Tower defense game Rhune has an unusual game mechanic. If a player wants the towers to deal a damage, he needs to play given melody to the rhythm.
... I played the v1.1 version and did not stop until I beat all the levels. I tried different strategies for tower placement, combining different attacks and even different ways to stay in the rhythm.
I liked the gameplay because I enjoyed the learning process to be successful. I find the game's graphics cozy.