Great game, even though I couldn't finish it. In the end there were 4 rooms I just couldn't seem to reach from any side. Considering x the horizontal axis and y the vertical axis, and starting from the lower left, I couldn't get to rooms x8y3, x8y5, x7y5 and x7y6. I could see the key on room x8y4, but couldn't reach it. From looking at the map I believe the entrance to that area was trough room x6y6, which I had visited before, but couldn't get back to after the last expansion. I guess I didn't collect all the keys in the most intuitive order, so I wonder if that made an area I should have visited earlier unaccessible later.
Also I did grab a heart through a hidden passage while trying, with no sucess, to get to the last key. Other than that I thought it was a great little game, wish I had finished it.
By the way, on the design of the character I see inspiration from the tiny pixelated sprites on the map screen of the DS version of N+, am I right? I always thought those sprites were pretty cool.