Good game. Here are my pointers for great
Naturally this is my opinion so naturally it is subjective.
I am a fan of "per turn" games and I think you did a great job here. So lets say that you do not really need to improve the concept when you do the second one (hopefully you will). However once you get a successful strategy with your character you need nothing else to win the game, independent of which creature you are fighting. A couple of creatures with different attributes (such as resists, etc) would have forced me to come up with different strategies depending on the arrangement of the enemies throughout the dungeon.
The last level was rather disappointing since it had no real boss, and most enemies were distributed one per room, which made it rather easy. I also found the game short which is a good sign in the sense that it leaves you wanting more, but still adding to the disappointment.
Also I would like to point out that I disagree with your classification of who is the easiest character: the fighter definitely did not work for me; the wizard for some reason leveled really slowly in comparison (I think it is associated with the fact that I killed most creatures with an opening fireball... I don't know if that makes a difference but you should look into it). The one that you said would be the most challenging turned out to be the easiest for me, i.e. the thief. Comparing with other reviews below it seems that the one which is the easiest to use is merely a matter of taste and gameplay, which again makes your game a successful one for allowing disparity in character preference.
So, I look forward to number 2 and hopefully it will be even better.