They're COLUMNS!
I haven't received any "can't find them' comments yet, but realising that some will download the first castle set from the main page (which won't show the detailed description), I thought I'd best tell you that the castle towers are COLUMNS.
If you place a floor tile on top of the column, it is concealed in the tower, but it makes the tower stackable without using any build cheats. This also applies to the quarter columns of course, and it also makes them walkable by Sims (should you need an extra corner tile on your rooftop).
There hasn't been any comments yet, be the first to post one!