Although this game is a good time waster, it's not exactly fun to play. I have no real issue with the default class, as it plays like I would imagine a basic class. However, the other classes that I've unlocked can use improvement. For the priest, if you're gonna make his heal cost 10 coins, then make it heal a little more. The archer... is just atrocious. His arrows cost gold and can't kill the enemies that are considered too strong to kill with normal attacks, yet his knife is just the same as the knight/squire's; not to mention the arrows don't hit the slimes from far away: you have to be at point blank. When I first unlocked this character, I literally couldn't kill anything with it, and his arrows costed so much gold that it took way too long to unlock the knife just so I could kill things and keep my money. The monk is... alright I guess, but his unique attack was useless when I first unlocked him. Well, useless except for the beginning portion of the game. Otherwise, he's just really weak, especially when you start to encounter more and more powerful enemies that can't be killed by basic attacks. I haven't unlocked the other classes yet, but you should probably do something about the ones I've mentioned.
For the shop, it's alright for the most part, but the randomization is frustrating at times. Particularly when it offers me items that I don't need. You should make it that it always gives you the opportunity to buy healing items and make the other two randomized; in my opinion at least.
The Upgrade system: misleading. What I mean by this is that the upgrades don't really feel like upgrades (for the characters I've unlocked). This is especially so when I'm spending money on abilities I've already unlocked in another class. Perhaps you should make it that some upgrades are carried on between classes so that way you can make some more creative abilities. This would also make it that, when you change classes in game, you won't die immediately (seriously, I've died shortly after every initial class change because I no longer had abilities I've already unlocked... wtf, man? wtf?).
Anyways, as I've said before, this is a good time waster; but in its current condition, it isn't what I would consider a fun game to play. However, that doesn't mean you totally suck at game making. There are just balance issues and various other problems to fix, like with any other game.