Is the gray piece surrounded by the red pieces?

Yup, see you can hit test coordinates. Your cathedral has 5 blocks that can touch red. There are 18 possible positions for surrounding red blocks. You can hit-test all of these coordinates pretty easily.

See what I mean? Like, cathedralMC would test (x,y):

(0,25);(10,25);(10,15);(20,15);…etc.