I think it would be great to put more emphasize on the arrow. For example, rapid-firing but small damage versus slow-firing but high damage. The current unchangeable (the upgrades doesn't really help) fire rate of the arrow is awful. Other than that, great game all around.
PRO TIP: max out the archer, the cannon, and the witch. Move SLOWLY in TIGHT group (tip: keep clicking 0 to keep your party in one tight group). KEEP A SAFE DISTANCE. Unless a dragon is on your way, you're unstoppable. Your melee cards doesn't even need to break a sweat.
There are games that 'has potential'. There are games that had achieved those potentials and go beyond. Kingdom Rush is example of the latter. Great gampelay, graphic, voice-over, and the little details that make this game even more lovable. This is the best tower defense game I've played so far. Or flash game, for that matter.
 "Sentry Knight"){kind=icon}
 "RPG Shooter: Starwish"){kind=icon}
 "Epic War 3"){kind=icon}
 "Kingdom Rush"){kind=icon}