Why 8" fieldstone tower? so you can make 4" round corners for your modular dungeon.
There allready is a 6" fieldstone tower, for the people who use a 3"x3" grid for their dungeon.
Same applies to gothic.. but gothic is lacking the 6" tower, so the gothic dungeon builders who use a 3"x3" grid can't make rounded corners.

If you'd do a 8" fieldstone tower, I'd like to request a rounded 0.5"x0.5" block, so it can be used for the inside-corner wall on a dungeon module (2 versions would be even better, particularly if the 2nd is slightly slanted to give it more contact space on the bottom).