Perfect square holes

I have walked through the castle in Canafis lots of times before it hit me. The holes in the floors where the spiked chains run through are perfect squares! It's a mystery how this can be; the castle is falling apart, loose floor boards, crumbling walls and smashed up furniture. And in the midst of this rubble are those perfect square holes. No crumbling edges, no round corners, just smooth squares! There must be a secret behind their form, but at the moment it escapes me...