about scheduling attack and synchronizing it with players, maybe something like scouts reports?
like "There recently has been reported an activity of mutants in the vicinity of your base, you should expect an assault soon"
soon- here it would be from 30 minutes to 2 hours for example. It would give players time to assemble, position and rearm mercs and prepare for the attack... or empty the base, sell it to some suckers, whatever other strategy. Of course attacks should at some "civilized" hours. Or, if it would be possible, there could be a programm that would gather statistical data about bases- and bases wouldbe attacked at hours when they are ussualy the most populated.
how do you like this?