This was awesome. At first I was skeptical if I should agree that this is even a metroidvania... until I realized that the rooms you acquired were your skill system that assists in maneuvering the game. You literally earn spaces to move around the level. This is a great spin on metroidvania, you should turn this into a larger game, it's got real potential.
By the way, I feel that providing restrictions on where you could place a given room would be a great addition. There is a lot of puzzle potential in that aspect of the game. Perhaps rooms that can only be placed once, and other hubs to tunnel toward. Also, perhaps the difficulty of a room should also vary based on where it is placed. The obvious idea is increase difficulty as it is placed nearer the goal.