Balancing is one of the most important parts of a game. Having good balance in a game allows for the player to have fun while having a challenge. In TD games, the towers have to be generally pretty balanced. They shouldnt be completely shredding the enemies to where there is no challenge or dealing so little damage to where it is impossible to survive more than a few waves. Lets be honest, TDS didnt always have a good balance to the towers. Some towers were just ultimately useless (cough cough freezer) and some were just completely nerfed to the ground (cough cough freezer). Of course they have been working quite a bit on fixing these, over the past year they fixed things like militant, mortar, and electroshocker, as well as added the beta version of redesigned gamemodes for some maps. Of course they still have work to do, but they have been improving. Towers like turret and accel now boast things like higher prices and overload, making them slightly less strong yet more fitting for the high placement limit (in accel's case) and more fitting towards making the game more difficult.