CharlesGrey: You're at the evil wizard guy, right? If you think this is the end of the game, you're in for a surprise. :D
ZFR: Yes, I was beginning to suspect it, since it feels too short otherwise... And my star count says 35/130. And I haven't had all heroes unlocked.
I just wasn't sure if the extra stars and heros weren't something you'd get from DLCs or something similar.
So there are more levels then? Good to know.
Ew, no, as far as I know the game is complete. No additional purchases required or anything like that.
You'll unlock a ton of extra missions after the "end boss", including the path to the
real end boss. :P
With the optional challenges included, the game is quite long. I wasn't even going to bother with them at first, but once you beat a few, it's kind of addictive... I think I'm at 125/130 or so now. Haven't played in a while, but sooner or later I might try to get those remaining few stars.
If you just can't beat some missions, including the boss fight, you're probably placing or picking your towers inefficiently. Make sure you place your most powerful towers ( artillery etc. ) in a way which maximizes enemy exposure to their attacks. And don't under estimate ground units -- they hardly deal damage, but you need them to keep enemy troops close to your attack towers for as long as possible. Barbarians are also an interesting choice, since they can block enemies, while also dealing decent ( ranged ) damage. And during boss levels, make sure you have enough damage dealing towers placed along their movement path. ( Trial and error involved again, since you won't know their exact path on the first try. )
Also, if I remember correctly, you can adjust the game's difficulty setting. If you get frustrated on Normal mode, you could try Easy, but I'm not sure how exactly it affects gameplay. Probably less enemy troops, and lower health on boss enemies.
Edit: Never mind -- I just saw your remarks regarding difficulty. Haven't tried Hard mode either, since even on Normal some levels are brutal. :D