I've used The Keep as a home base for the players for the past few games. After running through some "From The Ashes" Greyhawk and then a few good MERP modules (Dorwinion Wine Merchants!), they found the Keep. It's on the edge of the Vesve Forest, so there's usually something going on.
Being on the boarder of civilization, they don't always come home to peaceful times. Politics, Lizardman peace talks, spiders in the forest, invading orcs..
The entire Caves system was a holding area where troops could stay while awaiting orders from the Underdark. How else could so many hostile races live so close together? So, it was more of a barracks, with each section devoted to different creatures. The evil Priests gave them their orders; "build forces and raid specific outlying lands".
Plenty for the PC's to do.
Funny thing is, almost every player has played and GM'd the Keep at least twice over 20-30 yrs of gaming. But it's all new with RM rules.