Nice mix of the tower defense and bubble shooter! I like that two games are not separated, and towers working as bubbles adds a good amount of depth. It creates a moment of choice such as 'I want to destroy this bubble but doing so will lose me a tower'.
I found out spamming cannons is the best strategy, maybe you could give more incentive to use other types of towers. One example idea is to make a tower more expensive each time you purchase that tower.