Gargoyles are creatures that are animated out of stone. With wings on their back, the Gargoyle is allowed to negate it's heavy weight and fly to where it desires. These monsters can be found at the top of Slayer Tower. Due to physical nature of the monster, this creature only attacks with melee, and would be weak to crush attacks and water-based spells.
A Gargoyle's hitpoint bar will visually refill to fully green after the player's killing strike, despite being killed.
Gargoyles are most likely located at the top of the tower to show a resemblance with it's monumental qualities.