Memory leaks. It isn't about speed at the later levels, but losing an additional 20k mem/sec to the flash plugin. Flash shouldn't be using 400mb of ram by lvl 90.
Damage notifications pop up in the worst possible place- which is right over the image of my sheep. Very tough to navigate sometimes with that going on.
The best characters are the necromancer, cause he can summon and heal. The chicken cause he can fly, easy to get and possible to kill any unit. Bahamut cause he has a 3 direction attack, can fly, and has 110 hp. If you are trying to get the annoying units that can change you the best way to get them is be a level higher or low with an attack that goes up or down (bahamut) or shot them continuously in the back. If you are looking for the last row of units they seem to only spawn on the level that starts with you dropping down a long shaft. Go to the right till you see 2 dark elves, then fly up.
Nice development effort. Recommend a way to maybe track current quests and perhaps change the death gate creature spawn from a % life of the gate to a timer. The ambush just after the first town is a bit overpowered for that current character level. Use 3 mobs instead of 4?
A few things would help this game along. 1) Hidden menu during gameplay. 2) Ability to import images from the desktop for custom puzzles helps replay value.
Otherwise a very nice puzzle game!