Some castle keeps actually had rectangular towers, so don't rule that out.

Molds I own:  
Fieldstone: 57, 58, 59, 70, 71, 75, 77, 203, 220, 221, 260
Cavern Tiles: 83, 84, 85, 266, 281, 282
Gothic: 41, 42, 43, 44, 45, 205, 235, 240, 250
Sci-Fi: 270, 272, 273, 301, 302, 320

Wishlist:
Cavern: 81, 82
Gothic: 40
Egyptian: 93 - 99, 292
Sci Fi: 271, 325 - 327