It's not quite traditional tower defense, but Bad North is great, and scratches the tower defense itch for me.

It shows the map and enemies you will be up against each level, and if you lose it's always clear why. (sometimes it's because you got greedy or were stretched thin and didn't send enough units, or because you didn't choose to send the right ones, and often it's because you made a tactical error while fighting. But it never feels like the game is being randomly unfair)