Game is nice, i like your idea. But is to stressful, cause we have to get in both sides to attack.
Solutions for this will be multiplayer, like a cooperative game. Or try to spawn other mobs, from tower to enemy, like helpers. And add a option to upgrade them. Cause is no time for player to have a breath. Or make some upgrade shooting cannons. I don't like , when wave spawns, is no extra gold on mobs, is same amount, or i think is to low. Cause is hard to upgrade.
Anyway you did a great job, like your game. Thank you for letting me play it.