BFP-109 "Training Day" uses the castle overlay BFP C-1 with the following SBR: "Building oQ4 is a one story building with Rooftops (B23.8) and Cellars (O6) in play; ..."
At what level are the Rooftops in this case?
Because Rooftops are normally only available in buildings with more than one level, determining their height is relatively straightforward: it's a half-level above the top-most level. A two-story building would have it's Rooftop at level 1.5 (a half-level above level one, the highest level); a building containing a playable level 2 would have its Rooftop at level 2.5, and so on. This conveniently even means that a building's obstacle height is equal to where its Rooftop level is, which makes intuitive sense.
The castle on C-1, with the SBR above, is a bit trickier. Are the Rooftops at level 0.5 (relative to the building itself, netting out the level 3 hill), since the highest level is level 0? Are they at level 1.5, a half-level above the obstacle height of the building? Are they at level 1, at the obstacle height level itself? The latter is the most intuitive, and I believe is the intended effect, but the SBR should probably be errata'd to be clear about the height. The ASL rules as written do not allow for a clear resolution of this particular SBR.