I've been reading Contamine's War in the Middle Ages and there are a few times when he refers to some sort of siege weapon called a "cat" - something rare enough to warrant individual engines being given nicknames. But I've never seen such a weapon depicted or mentioned in movies or books.

What was a cat, how was it constructed and used, how effective were they, and how did people defend against them during sieges?

EDIT: I may have goofed in phrasing this question. There seems to be some thinking that Contamine is referring to "catapults" so let me quote the relevant section (note this book is originally in French, I have an English translation; bolds are mine):

[Preceding section largely about digging] ... above all, the use of approach engines, various types of counter-fortifications which allowed them to continue to injure the enemy while coming up to the walls. These tower-machines, belfries, or wooden castles, often depicted in manuscript illuminations, bore many names, some of which had been handed down from Antiquity or had been rediscovered in specialist Latin works: sows (truies), pent-houses (vinae), cats (chattes), weasels (belettes), sentry-boxes (guerites), cover ways, cats' castles {Footnote here: "Vineas et catos versatiles que sunt machine belliche, says John of Garland in his Dictionary"}. Sheltering archers, knights and crossbowmen, the majority of these engines could be placed on wheels or rollers to be pushed close to the opposing walls by the dozens and dozens of labourers. Smaller ones were erected on carts. At the siege of Beaucaire by Simon de Montfort in 1216, the crusaders' leader had carpenters construct a 'castle and a cat' (castel e gata) out of wood and iron but also from leather, this latter material being employed without a doubt to protect the engine against risk of burning. The contraption was stationed between the ramparts and the ditches, right up against the wall, and it was guarded day and night. Two years later at the siege of Toulouse the same Simon de Montfort employed another cat which the Song of the Albigensian Crusade describes as a redoubtable monster sheltering 400 knights and 150 archers and fearing neither trebuchets, stone-throwers (petrariae) or rocks because of the iron and steel used to bind its different parts together - the platform, sides, beams, cross-pieces, doors and roof. When he was besieging Damietta during the Seventh Crusade, Louis IX 'had two belfreys made which were called cats' castles for there were two castles in front of the cats and two houses behind the castles to protect those who mounted watch against the attacks of Saracen engines.

So in this section, I get the basic outline of what it is, but not really what it does besides house a lot of soldiers. And the terminology is confusing because in one description it insinuates a "cat" is just another name for one of the "tower-machines," but the Damietta account seems to suggest it was a specific thing.