There's some stairs leading down to the first floor.
From the tower you observe the keep's inner courtyard. It is made of hard-packed earth and has an eerie air of desolation. Jagged boulders emedded in the ground seem to have been dropped here, or thrown from a great distance, many of the walls bear large dents and cracks where these boulders might have one struck. Two massive skeletons lie at opposite ends of the courtyard, one propped up by the watchtower and the other sprawled at the far end by a building that could be a stable. To the south, where a section of the keep's outer wall has collapsed, a third giant skeleton lies partially buried in the rubble. From above, you can see there's a hole in the roof ot the south-east building.
(overhead map)

You're on the upper part of the west-south tower, aka the room on the left side of the map. Going down the stairs will take you to the blackened room below on the "main" map.