Medieval Siege Machines: The Bellifortis by Conrad Keyser

medieval siege machines

One of the most imaginative and fascinating works to depict medieval siege warfare is the Bellifortis by Conrad Keyser. Born around 1366, Kesyer was originally a physician until he developed an interest in engineering. He became a mercenary, and fought at the Battle of Nicopolis in 1396. This battle was a heavy defeat for a crusader army, and Keyser blamed the loss on Sigismund, king of Hungary. Afterwards, he worked for Sigismund’s brother, King Wenceslas of Bohemia. However, Sigismund overthrew his brother in 1402 and forced Keyser to go into exile.

Ill and expecting death, Keyser decided to write a book about medieval siege machines and military matters – the Bellifortis. He had the good fortune of having some German artists travelling through the same town he was in, and Conrad hired them to add illustrations. Four early drafts of the Bellifortis were made, and while the work was left unfinished after Keyser died in 1405, it became a popular work for those interested in warfare. Here are ten images from some of the fifteenth-century manuscripts:

Conrad had an image of himself included in one of his manuscripts – it was very rare for an author to include a portrait of himself in a medieval work.
An iconic weapon from the Middle Ages, the trebuchet was used to fling stones (or anything else, including people) at the walls of a castle.



One of the most ancient weapons used in siegecraft, it might not be surprising to see this kind of decoration used on a battering ram. Having that kind of weapon being brought against you might make any castle defender fearful
Gunpowder weapons were only just beginning to be used by the early fifteenth-century. This image shows a small cannon being fired.
If besiegers wanted to get close to the castle’s walls, they would need protection. This scene shows men walking underneath a type of cover, while the castle’s defenders try to throw stones down at them.
As well as including images of the larger weapons, Keyser had his illustrators draw many of the smaller pieces and tools that went into creating siege machines. Here we can see several types of bladed items, such as scissors, that would have been useful for a military engineer.
Keyser had several drawings made of different types of siege towers.
One common way for attacking a castle is to mine underneath. Using teams of miners, the attackers would dig under the walls, keeping wooden supports up to prevent the mine from collapsing too early. Once the besiegers were ready, the wooden supports would be set on fire, which caused the castle walls to collapse into the mine.
A pontoon bridge might be useful in crossing a river or moat.
If one cannot break through a castle’s walls, another option would be to get over them. This rope ladder would allow attackers to climb to the top, but they would be vulnerable to the castle’s defenders, who could shoot arrows or throw rocks at them.

The medieval historian Lynn White Jr. finds that the “Bellifortis was written by an immensely complex man whose mental processes seem enigmatic to the twentieth century. But he was typical of his time, and the edition of this book will be a rich mine for historians, not only of technology, but also of art, costume, magic, and much else.”

Benedek Lang adds,

Whether it was to be used as a manual in military conflicts or simply to entertain its peaceful reader with its technical, literary and magical material, is still a matter of debate. In any case, it describes and depicts a number of martial instruments, ladders for the siege of castles, machinery to help horses cross a river, catapults, rockets, arrows, arbalests, nails, scissors, clasps and horseshoes, and recipes for preparing various kinds of fires. The reader is also shown some – at least in the context of engineering – unexpected pictures that seem to be somewhat less related to military affairs: a female chastity device, a tool for castrating men, the black queen of Sheba, a goose fastened to an anchor, and a few pictures on how to prepare a bath appropriately. Some of the machineries bear fantastic features; among them a huge wheeled cat with a long destructive pike in its mouth that might have served to frighten the enemy.


You can see three fifteenth-manuscripts which have the Bellifortis:

Besançon – BM – MS.1360

Universitätsbibliothek JCS Frankfurt am Main – Ms. germ. qu. 15

Talhoffer Fechtbuch – MS Thott.290.2º 

To read more about Conrad Keyser, see:

Lang, Benedek, Unlocked Books: Manuscripts of Learned Magic in the Medieval Libraries of Central Europe (Pennsylvania State University Press, 2008)

White Jr., Lynn, ‘Kyeser’s “Bellifortis”: The First Technological Treatise of the Fifteenth Century’, Technology and Culture, Vol. 10, No. 3 (1969)