a way for defences to gain "exp"

What I think is first, all defences have a kill count. You can check the kill count by pressing info on the defence. Next, every kill the defence gets,like every balloon an air bomb has destroyed, or every archer a cannon has killed, etc, will take one second off the next upgrade time, so if a cannon has 60 kills, and the upgrade takes an hour, the upgrade would then only take 59 minutes. It isn't op, and it would help begginner players join a league faster. If the troop that the defence kills is level 2, then two seconds get taken off the upgrade time. If level 3, then three seconds, etc. The one fault would be mortars shaving minutes off the upgrade times, but you could just make the upgrades for mortars take longer in general. Comment anything I missed, and this is my first post, so tell ke what you think!! Thanks for reading!!!