PCs on the Battlefield: the Siege of Cartagena

The 1741 siege of Cartagena, in what is today Colombia, featured a really interesting variation on the normal siege framework. The British attackers knew going into it that they would likely suffer a devastating outbreak of yellow fever, and that the Spanish defenders would likely be unaffected. Thus, they were racing the clock: they had to breach the walls of Cartagena before they lost so many men from disease that the feat became impossible. The Spanish had the opposite problem: they were fighting not to keep the British out, but merely to slow them down long enough that yellow fever would win the siege for them. The affair makes a great template for a siege in an RPG because it presents a ticking clock that accelerates and a siege structured around chasing a series of attainable goals, not cracking a single impossibly tough problem.

This is part seven in a continuing series about PCs on the Battlefield where I look at human-scale engagements where individual PCs can turn the tide.

This post is brought to you by beloved Patreon backer Joel Dalenberg. Thanks for helping keep the lights on! If you want to help keep this blog going alongside Joel, head over to the Patreon page – and thank you!

Image credit: Jorge Láscar, released under a CC BY 2.0 license

Yellow fever is originally an African disease; the virus and the mosquito that carries it almost certainly came to the New World aboard slave ships. If you catch the disease as a child, you’ll probably be fine. Catch it as an adult and you’ll probably die. Critically, once you survive yellow fever, you are immune to the disease forever. That means people who grew up in areas where the disease was endemic (as it was in the Caribbean by the mid-1600s) could live their whole lives without fear, while visitors to those fair shores historically died in droves of the vomito negro: black vomit with the consistency of coffee grounds.

These particular characteristics drove yellow fever to play an important role in the history of the Caribbean, one recognized at the time by the people it affected. Unlike most diseases in war, yellow fever took sides: it favored the defenders. While it arrived too late to help the Caribbean’s indigenous nations, from the mid-1600s on, defenders could count on a ‘fifth column’ of pestilence. The defenders, after all, had grown up in the Caribbean. They’d already had yellow fever as children and were immune. But the attackers – tens of thousands of adult soldiers from Europe – faced a foe deadlier than musket fire.

Spain in particular, with its large immune New World population, relied upon yellow fever as part of its military strategy. Few other powers bothered to build forts in the New World, since a fort only helps if you anticipate a relief column will be sent to help you. Other colonial powers knew that by the time word reached Europe of a siege and reinforcements could be sent, the siege would have been over for months. Trans-Atlantic travel was too slow for forts to be worth the expense of building them. But the Spanish knew they could count on a relief column of yellow fever to wipe out their besiegers. So Spain invested heavily in fortifications in its Caribbean possessions.

Image credit: Bernard Gagnon. Released under a CC BY-SA 4.0 license.

Those fortifications proved their worth during the colorfully-named War of Jenkin’s Ear (1739-1748), itself little more than the Caribbean theater of the War of the Austrian Succession (1740-1748). This was another of the interminable, indistinguishable European gunpowder wars I’ve already encouraged you to gloss over. So we’ll skip the casus belli and jump right to the part where Britain decided it wanted to conquer much of Spain’s empire in the New World.

In 1739, Britain put together a considerable naval force to send to the Caribbean. But they couldn’t find any naval officers willing to lead it. Fear of yellow fever was so great that units would desert en masse when they heard they were bound for the West Indies. Lacking anyone with recent experience to head the expedition, the government tapped member of Parliament Edward Vernon. To his credit, Vernon had been a captain in the Royal Navy – eleven years earlier. Also important but unacknowledged, Vernon had served in the Caribbean before and had survived yellow fever, making him immune.

Vernon first sailed against Portobelo, in Panama. The city lay on the Caribbean end of a road that crossed the isthmus, a mule-powered forerunner to the Panama Canal. The tiny garrison held out for two days. Vernon stuck around long enough to knock down the fortifications (in case later a bigger garrison moved in) and got out of there. He sailed for Jamaica, a British possession and safe home base, since its mountains are high enough that anyone in them is above the elevation yellow fever mosquitos can tolerate. Vernon got word his ‘great victory’ was being rewarded with reinforcements from home. These thousands of additional soldiers were commanded by Thomas Wentworth, an inexperienced and unqualified officer. Vernon and Wentworth together led the expedition against its next target: the wealthy port of Cartagena, in what is today Colombia.

Image credit: Martin St-Amant, released under a CC BY-SA 3.0 license.

Cartagena was a tougher nut to crack than Portobelo. Its decent-sized garrison was commanded by Sebastián de Eslava y Lazaga, the viceroy of New Granada and a career soldier. Its small naval defense was commanded by Blas de Lezo y Olavarrieta, a career naval officer who’d lost one eye, one leg, and one arm over the twenty-one battles he’d survived.

Cartagena’s extensive fortifications – recently upgraded for just such an event – were different from those the English were accustomed to facing in Europe. Those were massive unitary structures. But Cartagena’s fortifications were designed around ‘defense in depth’: a series of walls, forts, bulwarks, and batteries. Each could be individually defeated. But after you took this fort, then you’d come to the next one. And the next one. Cartagena is built far back in what was – at the time – a narrow bay, so to take the city you had to take each of these defenses in turn, and that slowed you down. If the city had one single spectacular wall, an attacker might get lucky and take it in one go. With defense in depth, an attacker was likely to have to take a long time, cracking each defense in turn. And that bought enough time for yellow fever to set in.

Vernon and Wentworth tried to land before the summer rainy season started. No one knew mosquitos spread yellow fever, but everyone knew it came with the rains. The British didn’t quite manage it. They arrived in late March. It took mere days for a handful of men to display symptoms of yellow fever.

Disease – as we’ve all become painfully aware – spreads exponentially. A few men caught yellow fever from mosquitos that had previously bitten infected monkeys in the hinterlands. The blood of those men filled with the virus. Other mosquitos then bit those men and drew the virus inside them. When those mosquitoes bit other Englishmen on later nights, the insects’ tainted saliva infected those men. The more sufferers there are, the greater the chance a mosquito will encounter one and spread the disease. Thus, the infection builds. Every day, the British lost more soldiers to disease than the day before. This meant cracking Cartagena’s series of forts was a race against time: the British had to finish the job before they lost so many soldiers that taking Cartagena became mathematically impossible.

Image credit: Bernard Gagnon. Released under a CC BY-SA 4.0 license.

Wentworth’s soldiers landed on March 20th, 1741. They first faced a collection of forts and batteries, the largest of which was the Castillo San Luís de Bocachica, Wentworth moved slowly and orthodoxly, silencing the smaller forts and batteries so he could land more troops and artillery, and relying on naval gunfire support from Vernon’s ships. After five days, Lezo considered abandoning San Luís and saving the men inside. He decided against it, trading Spanish lives for time. It took sixteen days for Wentworth to breach the fort and storm it. During the Spanish defenders’ planned retrograde, they scuttled merchant ships and blocked the channels into the bay. It may not have looked like it, but this was a big win for the Spanish defenders. Vernon and Wentworth took far too long to storm San Luís. Fever was spreading among the ranks.

In early April, the British entered the bay. Their next obstacle, before the formidable walls of Cartagena itself, was the Castillo San Felipe de Barajas, atop a hill overlooking the city. San Felipe was outside of the range of Vernon’s shipborne cannons. By now the fever was taking a great enough toll on Vernon’s crews (mostly in the form of those incapacitated on their way towards eventual death) that Vernon refused to loan any sailors to Wentworth as reserves. And in the chaos of landing 6,600 troops in the bay, the soldiers’ tents never got unloaded, so the English had to sleep in the elements, increasing their contact with mosquitos. In mid-April, hundreds of British soldiers died daily – almost all of fever.

Wentworth soon reached a shit-or-get-off-the-pot moment. His artillery still hadn’t battered a hole in San Felipe’s walls, yet if he didn’t act soon, he wouldn’t have enough soldiers left to actually take the fort even if they did make such a hole. So he found some local guides, had his soldiers build ladders, and on April 20th sent them through the woods towards San Felipe. There were 1,500 British attackers (Spanish sources give 3,500) against 250 Spanish defenders. Yet the guides led the attackers astray, whether out of loyalty to Spain or total incompetence. Those attackers who reached San Felipe found their ladders were too short to get over the wall. In the resulting fight, the Spanish killed 179 British and wounded 475. The British killed 2 Spanish and wounded 13. But again it was the fever that was the real killer. By April 21st, of the 6,600 soldiers Wentworth had landed to take San Felipe, 2,500 were dead and 900 were incapacitated, leaving 3,200 (48%) fit for duty. Five days later, the British were down to 1,700 (26%) fit for duty. See how that exponential curve takes off?

The walls of the Castillo San Felipe de Barajas

Image credit: Bernard Gagnon. Released under a CC BY-SA 4.0 license.

The siege was effectively over. On April 28th, the surviving British soldiers boarded Vernon’s ships. The fleet spent eight days destroying the fortifications near the mouth of the bay, just in case Britain decided to come back. The British and Spanish also exchanged prisoners. On May 7th, the fleet set sail. Of course, just leaving Cartagena didn’t mean the epidemic was over. The species of mosquito that transmits yellow fever had laid eggs in the water casks of ships in the fleet. On the return trip to Jamaica, another 1,100 soldiers died. The dead were thrown overboard to feed the sharks.

The four commanders at Cartagena all survived to the end of the siege. Lezo suffered wounds that took his life in September. Eslava lived another 18 years and enjoyed a successful political career. Both received titles of nobility from the King of Spain. Vernon and Wentworth stopped talking to one another, yet went on to launch failed attacks with their ever-diminishing forces on both Santiago de Cuba and Panama. They coordinated their planning via letters. In 1742, they called it quits and went home.

The body count at Cartagena was almost impossibly lopsided. The Spanish lost between 200 and 600 soldiers. Nobody mentions yellow fever among them. Counting reinforcements and replacements, Britain sent about 14,000 soldiers to the Caribbean in 1739-1742. 71% died in the process, only 6% in combat. The core group that sailed first from Britain (and thus was exposed to yellow fever the longest) suffered 90% mortality. We don’t have hard statistics for the British sailors, but evidence suggests their mortality was comparable. J.R. McNeil (see ‘source’, below) estimates that about 21,000 of 29,000 British sailors and soldiers died of disease, plus about 1,000 from other causes. Neither Vernon nor Wentworth ever expressed remorse for getting so many of their men killed.

Image credit: Bernard Gagnon. Released under a CC BY-SA 4.0 license.

The siege of Cartagena provides one of the best templates I’ve yet seen for running a siege at the table that’s actually interesting. That’s because the danger of disease provides a countdown timer that accelerates.

In a fictional siege based on this one, I’d put the PCs on the side of the attackers. The attackers are more dynamic: they’re acting, not reacting. It can be a lot of fun playing a small group of defenders against a much larger group of attackers, but you generally want to wrap that kind of adventure up quickly. See this post for an example – one where I am, unfortunately, also encouraging you to emulate the colonial British. (That post is from 2017 – my posts sure are a lot longer today, huh?)

When implementing a siege based on Cartagena, suggest to your players that their characters were already infected and survived yellow fever (or whatever weird science-fiction disease you use) at some earlier point, either off-screen or before the campaign began. Ask them to tell you the circumstances of their illness. That way, the only people dying from disease will be NPCs; contagion is rarely fun to roleplay, and it sucks to have your PC killed by some arbitrary illness.

With a series of forts and batteries to be overcome, each is a puzzle that lets your PCs show off their neat abilities. Does someone have a chameleon cloak? They can infiltrate a battery by slipping in among its reinforcements. Can someone phase through stone? They can pass into the powder magazine (or energon cube storage or whatever fits your campaign setting) and blow it up. As long as your party has some magic or sci-fi powers, taking a series of forts and strongpoints provides a colorful playground to run amok in. None of these obstacles are terribly impregnable, since that’s not the point. Remember to keep the clock ticking. Mopping up a seized fort takes time. Moving the artillery up takes time. Planning the next scheme takes time. Provide good reasons why several days must pass between each wild maneuver the PCs attempt.

On each day of the siege, roll a number of d10s equal to how many days the siege has endured. So on day 5, you roll 5d10. On day 12, you roll 12d10. That’s how many attacking soldiers die of disease on that day. On average, by day 10, they’ll have lost a total of 303 soldiers. By day 20, they’ll have lost 1,155. The number of dead should double every 8 or 9 days or so. Unless you played a lot of World of Darkness, you’ll probably not want to roll fifteen d10s, but you can offload that onto a dice-rolling app or just type ‘roll 15d10’ into Google. Wentworth and Vernon called off the attack when they were at half-strength. With this technique, half-strength will arrive after 19 days for a force of 2,000, 30 days for 5,000, and 43 days for 10,000.

In mid-January I started the last semester of my Master’s degree in GIS (Geographic Information Systems – fancy computer maps). This semester is weird. Because I started my degree in the summer of 2020, it’s my first semester in the classroom rather than taking courses via Zoom. This is my first time taking college classes in person in a decade. And because my classes have both grad students and undergrads, I’m sitting beside people two-thirds my age. I’m not quite old enough to be anyone’s father, but it’s close. The weirdest thing is that it doesn’t feel weird. I expected to feel like some gray-haired old geezer. But instead it just feels normal, like the academic part of undergrad all over again, but with masks. Bizarre.

I’m also working while taking classes, which doesn’t leave me much time for Molten Sulfur stuff. Fortunately, I maintain a pretty robust buffer of blog posts. As of this writing, I’ve got five months of posts ready to go. So even if I can’t write a single post all semester, there will be no interruption of service here on the blog! I hope I don’t need that much help – five months of buffer takes forever to build up – but I’m sure glad I have it.

Anyway, if you’re looking for a GIS bubba in the May/June timeframe based in St. Louis, Minneapolis, or remote, let me know via the ‘Contact’ link!



Check out my book Making History: Three One-Session RPGs! Three awesome historical one-shots with pregenerated characters and a very simple rules system designed specifically for that story. Norse Ivory is a game about heritage and faith in the Viking Age. A Killing in Cahokia is a murder mystery in the Native American temple-city of Cahokia. And Darken Ship is a horror-adventure starring junior sailors on a U.S. Navy warship who wake up one morning to discover they’re alone on a ship that should carry three thousand.

Source: Mosquito Empires: Ecology and War in the Greater Caribbean, 1620-1914 by J.R. McNeil (2010)


Everyone needs content for their RPG campaigns: adventure hooks, puzzles, NPCs, political machinations, combat encounters, and adventure sites. That’s what this site provides! I draw RPG content from real-life fact and folklore, then give advice on how to adapt it to your fictional campaign. I believe content that is grounded in reality (however fantastical) is richer and more vibrant, and your players will appreciate the difference.


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