Correct me if I'm wrong, but time of placement of a tower controls vulnerability times of that tower, correct?
If so, if you placed it at a time such that your going to be busy when its open, it's your own damn fault.
And if you take the land over at an inconvenient placement time (i.e. that was when it was open, so you took it), then just place some placeholder stuff, come back later, terminate the towers, place new ones and be prepared to wait those initial few hours to defend.