Treasure Hunter Achievement
Find 15 secret rooms.
This is pretty easy as the secret room aren't all that secret. Just pay attention to your map and look for rooms that are blocked by a wall. Once you find one just hit it with and it'll open.
There are many different kind of walls. Barrels, planks, bricks, fire stone, cement, etc. Just keep your eye out and run along every wall in every room.
You should have 15 in no time since the levels are randomly generated each time you play one (aka you'll never play the same level twice.)