Cats are brilliant defensive siege weapons in Stronghold 2, although have a very slow movement speed. They will protect nearby troops from all damage off enemy ranged units, and the Cat will take on the damage instead.
Cats will also protect nearby friendly units against Catapults and other ranged missile attacks, which Mantlets lack. The Cat has many uses, including protecting your units as they dig the moat as well as generally advancing forward towards the enemy.
Enemy units must destroy the Cat before they can actually attack your troops, and the more Cats you have around your troops, the harder this task is. Cats can absorb a large amount of damage off ranged units, although melee units can quickly destroy it. Fire, for example from burning carts, can also cause a lot of damage to them.
You can recruit a Cat from the rank of Earl. You will need 100 gold for a Cat, along with 2 peasants from the campfire before it can be recruited.