There should be perks that cost 1 million elixir for defense towers. You could only have one perk per tower but they are exchangeable for 15% more elixir. As following, one perk increases range, but lowers damage, one should increase damage but lower fire rate, and another should increase fire rate, but lower damage. You could also upgrade the perks so it adds 5% more power for all of the stats on the tower.
