I just killed the third Keeper - after somehow getting teleported up to the tower in the air.

There doesn't seem to be any way down now. Help? I tried sneaking (to slow my fall rate) along the wall, but this place seems to be in mid air.

If you killed the keeper, then I assume you are not actually floating in mid-air, but rather on top of the tower. If so, there is a teleporter pad on the highest level where the keeper starts out. Jump into the teleporter and you will go back down to ground level.