A stone turret (or just a hollow room, depending on options). Radius, height, stone size and crown size are customizable. The top part can be printed as 1 piece (for those who dont mind 45 degree overhangs), or as two pieces (if you don't mind more assembly required).
Uses the wedge function from http://www.thingiverse.com/thing:129904, copied in full.
I might decide to put a door in it...dunno, that's like, work...man.
EDIT (2MAR2015): Deleted extraneous code from the wedge() function. Added some comments to make it easier to figure out what's going on. Thanks to http://www.thingiverse.com/litgoddess/about, for renewing my interest in this thing.
EDIT (7APR2018): EdgE NL has made a much needed improvement: https://www.thingiverse.com/thing:2847584