As per title. In the Oblivion dungeon in the ‘Marooned Confinement’ section (second part) there is a giant two-part invisible wall blocking all access in open terrain.
I came across it in the tileset with a cell in the middle of a room surrounded by other cells; the cell in the middle of the room had a shrine pillar, and both sides of the shrine pillar were completely blocked off with giant invisible walls.
I’ve come across this tileset hundreds of times in other dungeons and it should be fully accessible.