Engaged creatures can take part in combat. You cannot attack your own creatures (but see ranged combat). Living creatures cannot attack undead creatures, otherwise anything goes. Once engaged an attack is induced by clicking on the cell containing the creature you wish to attack. If successful the defendant will be killed and the attacker will move onto the corpse.
It is possible to attack inanimate objects and growths, although you cannot get engaged to them. Be wary of killing growths with buried creatures beneath, the buried creature could be revealed.