This is largely based on the layout presented in Keep on the Shadowfell. It's an old abandoned keep that a cult has made a base out of. The goblins have been forced to make the immediate entrance their camp, since undead infest the lower areas.

Where the stairs go down there is a lower level. Then there are at least 2 even lower areas. So i figured the grates would be fine. In level 2 they have pit falls with water at the bottom.