Nice game, addicting and fun!
The base of the game feels really solid, however there are some things that could be finetuned or rebalanced:
1. I've played as fire golem. Slays everything until wave 33, where goblin shaman could not be killed simply because he is immune to fire and fire golem has no skills other then fire-based ones. Plus shaman has self-heal. Maybe need to change resists of the boss? Same stuff may apply to skeletons and ice golem since they have pretty much ice damage only.
2. Are crafted runes single-use only? i crafted some and they disappeared after one run. it's really tedious to gather mats to craft even low-level ones, let alone ++ ones. Not really worth it if it is so.
3. Archer(3 runs), ninja, wizard(3 runs), skeleton sorcerer and hunter are pretty fun. Melee classes not so much - i've played as warrior and dark knight and it's pretty hard considering slow speed and limited range of skills, i couldn't have enough defense and attack at the same time, got killed every time even before 1st boss.
4. Playing as wizard and getting cyclone skill(or how is it called, one that summons storm) crashes the game pretty fast, after several casts of the skill. Something "out of bounds" exception.
5. Skeleton knight's ice shield and paladin's god favor skills damage player on each cast.