For that boss, you have to move to the desert terrain surrounding the tower to gain back your strength. (I'm quite sure)
After that boss, there's a puzzle or two and some special enemies, but there's no finale boss as I never got that far.
Check out my Keltos RPG as it's very similar to Diablo3 but I believe it's way better. Keltos only goes up to the second boss however. Nonetheless though, you should have a great time seeing how you're an rpg lover. :D