I left that off intentionally. As a DM I was always looking for good maps, preferably with numbers, that I could fill up with adventures for my players. If someone added a legend or lots of details or something, I either went looking for a map that didn't have all of that or just deleted it and made my own. Since I didn't use any strange symbols or anything, just stairs and stained glass it makes it easy for anyone who wants to use it for a game to just start writing.