I have to admit, I'm leaning towards this: I experience it quite often with towers and civic centers in particular. Imagine you're 95% complete with the first tower in your base and it comes under attack from an early rush. Would it feel fair that you have a nearly complete tower, but never get the chance to use it to repel the attackers, because they keep hammering at it with their swords? Would it feel fun, would it feel realistic? IMO, no. I can see the logic of the other option, but if you do decide to go with that, at least evaluate it honestly and consider if it really makes the gameplay better.