Iron Crypticle

This will likely be one of your first achievements. Maps are random drops from enemies, and collecting one reveals the contents of all rooms on the current floor. Maps are extremely useful since they allow you to plan your route to ensure you go through arcade rooms and shops, but they unfortunately are completely random. That being said, you'd be hard pressed not to find at least one map per run. I was finding a map over every floor, just sometimes later than others. Your first one will unlock this achievement.