Mega Tiles are just what the name implies: really big geomorphs compatible with Dungeons in Blue tile sets. Each Mega Tile is a single geomorph covering four times the area of a standard-sized tile. These larger tiles allow bigger caves and chambers to be depicted, and also serve to break up the gridded feeling geomorph-based dungeons can sometimes have.
Mega Tile Thirteen is a crypt complex with caves leading away in several directions. The stairs leading to this complex are designed to match up with Mega Tile Twelve, giving you instant access to a two-level dungeon complex.