Tiny, Tiny Keep (SketchUp)
Here's a tiny keep I made in SketchUp, and the logical explanation behind it!
First, I had it in mind to make something tiny, so small it has no outer walls. In any event, the barracks area is large enough to comfortably sit about 6 soldiers and their gear, and then the sheriff has his own room. If the village is attacked, villagers would retreat to this keep's main entryway, and if the doors into it were attacked, they would retreat into the inside of the round tower...and...I dunno, wait around and hope they don't get killed.
The roof over the stairs from 1st to 2nd floors serves two purposes: first, it prevents rain from draining through the gap. Second, it prevents enemy archers from shooting anyone going up the stairs.
In theory, all of the windows could be shuttered and locked from the inside, but I didn't build any shutters into the design.
I might design a small dungeon for it - not a literal dungeon, but a way that any villagers could exit the keep and a) not take up room inside of it that the soldiers could probably use or b) get killed. Also, dungeons are great excuses for adventure! I'm thinking, a secret door in the sheriff's quarters that leads into the otherwise solid rock tower base?
It uses a lot of wood, so it'd be very susceptible to fire...maybe I'll build a small cistern into the "dungeon," so volunteer villagers can get buckets of water and pour them on the keep's standing areas.
Walls extend 3'-6" above the floor in the standing area, with the crenelations carved 1' back down. That way, defenders can hide reasonably well behind the stone, and get a very clear shot at any enemies they want to aim at. Floors are each 10' tall; the tower is 50' diameter, and the square building is a 50' x 50'.