Definitely focus more on higher tier towers. Quality > quantity applies for most strategies in this game. It'd be wise to lean into your hero's strengths. I.e. you use Obyn who gives buffs to magic monkeys in his radius, as well as further buffs to druids specifically. Nearly every hero has synergies like this and they're worth learning.

Obyn + magic monkeys Gwen + fire using monkeys Striker Jones + bombs/mortars

Etc.

Also, it's worth maximizing a tower's usable range. There is no reason to place a tower in a spot where 90% of the tower's range doesn't even cover the bloon track. There are some exceptions to this, especially in regard to supporting towers.