Cyprus, the Mediterranean island nation just south of Turkey, took centuries to gain its independence. The Greeks, Assyrians, Egyptians, Persians, Romans, Ottomans, British and others all took their turns taking over the island, and each left their mark on the archeological record. But in a ruined chamber in a castle on the western corner of the island, it may be more apt to say the invaders left a smear.

In 1191, during the Third Crusade, King Richard I of England invaded Cyprus and ordered that a castle be built on the island’s western corner in order to defend the harbor there. Called Saranda Kolones, the castle’s name refers to its many monolithic columns. But in typical tumultuous Cyprus fashion, the medieval castle was only used for thirty years before it was destroyed by an earthquake. By then, King Richard had sold Cyprus to Guy de Lusignan, the King of Jerusalem. Lusignan and his successors had other plans for expanding the island. The wrecked port was abandoned and the castle never rebuilt.

As castles go, Saranda Kolones had a pretty poor run. But two University of Cambridge researchers recently realized that, precisely thanks to the castle’s short use, a priceless treasure had been left behind in the Saranda Kolones’ bowels. One of the centuries-old castle latrines (read: ancient toilet), they found, was still full of dried-up poo. That feces, they thought, could provide valuable insight into what kind of parasites plagued the former residents’ guts. And because only 30 years’ worth of waste clogged the ancient sewage system, those parasites could provide specific insight into what ailed medieval crusaders. The researchers rolled up their sleeves and collected samples from the dessicated cesspool.

To rehydrate the ancient night soil, the team placed one gram of their sample into a chemical liquid solution. They used micro sieves, or tiny strainers to separate parasite eggs from the digested remains of the crusaders’ meals. They created 20 slides, and peeked into their microscopes to see what creatures the soldiers may have left behind.

The samples revealed 118 “lemon-shaped” Trichuris trichiura eggs–a type of roundworm commonly called the whipworm–as well as 1,179 Ascaris lumbricoides, or giant roundworm, eggs. A control sample of non-toilet soil they tested did not contain any parasite eggs, confirming that the eggs did indeed come from the toilet, they report in the International Journal of Paleopathology.

The study of ancient parasites, whether through old bones that reveal leprosy-causing pathogens or dried up leaves that elucidate the cause of the Irish potato famine, is a thriving field. In this case, the long-dead parasite eggs were pooped out by the crusaders using the toilet years ago. These species reproduce within human bodies, and go on to infect new hosts through egg-contaminated soil or food delivered courtesy of the host.

Heavy infection with either of these worms was no picnic. The authors write, first of giant roundworms:

The mature female then starts to lay about 200,000 eggs per day that can be fertile or unfertile if no male worms are present. Although a mild infection with roundworms is mostly asymptomatic, heavy burdens with Ascaris can cause intestinal blockage and abdominal pain in adults. Because children are less able to tolerate parasites that compete with them for nutrients in their diet, heavy infection with roundworms can cause nutritional impairment, vitamin deficiencies, anaemia and growth retardation.

And of whipworms:

When the females reach maturity they can release 2000–10,000 eggs per day. As with roundworm a heavy worm burden may contribute to malnutrition, stunted growth in childhood and sometimes mechanical damage of the intestinal mucosa, diarrhoea and prolapsed rectum.

The presence of these worms, the authors write, attests to the poor hygienic conditions the castle residents likely practiced and put up with. “Poor hygiene with dirty hands, contamination of the food and water supplies with faecal material, inadequate disposal of the faecal material, and consumption of unwashed vegetables fertilized with human faeces are some of the means through which roundworms and whipworms are spread.”

The worms also could have jeopardized the health of their hosts, especially during years of famine when both parasite and human competed for scarce nutrients from meals few and far between. Previous studies found that between 15 to 20 percent of nobles and the clergy died from malnutrition and infectious disease during the crusades. Although death records for poor soldiers are not available, the authors think it’s safe to assume that malnutrition probably hit the lower-ranking crusaders even harder.

“It is quite likely that a heavy load of intestinal parasites in soldiers on crusade expeditions and in castles undergoing long sieges would have predisposed to death from malnutrition,” they write. “This clearly has implications for our understanding of health and disease on mediaeval military expeditions such as the crusades.”

Before contemporary readers breathe a sign of relief that these parasites infested the guts of people living more than 800 years ago, it’s important to note that the giant roundworm infests an estimated one-sixth of all humans living today. As the authors write, “In modern times A. lumbricoides and T. trichiura are two of the most common and widespread intestinal parasites.” Other parasites continue to plague human populations worldwide, especially in developing countries. Who knows what the archaeologists of the future will find in the scum of your latrine?