I have been playing flashgames since 2002. So maybe my humble opinion might help you out.

The biggest problem with this flashgame is the experience/gold advancement. Gaining gold just takes too long. Many players will stop playing after the 8th wave, with only one weapon and spell upgrade.

The gameplay itself is mediocre. Nothing groundbreaking. However, it gets better later in the game.

Take care of your experience curve next time. Make it steeper and add more content.