ANOTHER easy way to balance out silly gate towers strategy, is to give “bonuses” on how much time remains. Almost every game rewards the player for completing a level faster, because a player who completes the level faster is THE BETTER player, not the other way round.
Hence, bonuses such as added gold, XP, medals, and most importantly, SKULLS, should be added based on how much time is left, then the silly gate tower strategy will automatically be balanced out.
For example, we can add 1 skull per second left. Hence, if someone completes the base with 60 seconds left, he will be awarded an additional 60 skulls.
Hence, if a player chooses to put 6 gate towers, making it easier to complete his base, he will risk having the raider to gain skulls from completing the base faster. This will add diversity to defense strategy.
Not only that, it creates a new layer and meaning to “having a better defense” because even if you can beat base A and base B, but because base B is much harder, it takes longer to beat base B, it will then finally mean it is worthwhile to ACTUALLY invest in defense.