Laggy but good
Needs to be a special achievement for killing final boss while lagged to 1 FPS. Once you run out of special arrows with those summoner guys around, you're screwed. They take forever to kill, and they respawn assholes as fast as you can kill them.
Recommendations to make this better ...
1) Forget money drops; just give player money based on enemy killed. It gets annoying dodging enemies AND trying to pick up money at the same time.
2) DO drop health potions and arrows. Arrow drops occur in survival mode, but would have been nice to see in campaign, too. Once you run out of special arrows toward end levels, you're really screwed and will either die quickly or give up as the game lags to hell.
3) Forget buying health at intermission ... just replenish health to 100% for free. However, maybe let the player buy healing potions that can be used during the fights.
5) Upgrades need balancing. Upgrading hero doesn't seem to make much difference (especially health, since after upgrading it you still seem to only have 100 health). Upgrading bow, though is ZOMGWTFBBQPEWPEWPEW! Seems pretty unbalanced. And upgrading cheaper bows ... fairly pointless early on. Buying arrows can carry you through until you have the crossbow, and even then it's only worth bumping it to 3-4 points then tossing all other points into the main magic bow.
6) Enemies need to do more than just follow you around. You basically circle-strafe them into a blob and just fire away. We've seen this in other games like this, and it gets old. Some enemies need to run off to find cover to shoot you. Others should retreat when injured. SOMETHING besides them all just following the player around like idiots.
7) Enemies should do more than just "attack". Summon enemy was good (and a royal bitch at times). But you could have a sniper enemy that camps hard to hit locations and snipes. You could have a healer that injured enemies rush off to. Buffs could drop and enhance enemies within their zone of influence (which would make the player hussle to try to destroy the buffing machine/thing.) There just needs to be more than just enemies running after player to hit or shoot him.
8) Likewise, player enhancement need to be more than just "do more damage". Area Effect arrows would be nice (explosive, ice/freeze, etc). Homing arrows perhaps. It would start ripping off Diablo 2, but it just seems enhancements that did more than just increase damage output would have been entertaining.
9) Harder bosses. The bosses themselves were not that hard. The minions running around made them difficult, though. The last boss would have been a push-over without his summoners.