At first I was very pleasantly surprised and thought "wow, finally a mahjong game that starts you off easy" - but then it didn't get more difficult. I stopped at LVL 8 as it became more of a chore than a challenge.
With the very limited variety of pieces you can stack them up differently all you want - it is not going to be more difficult. In fact, I found myself more than once ending up with 6 or more identical pieces left on the board - a clear indication that there is a lack of diversity.
This game would profit immensely from adding new pieces; new piece could be introduced every three levels or so. The combination of added pieces and ever more complicated stacking could make for an intriguing challenge.
As it is now, it's a solid 2 stars.