I went ahead and added retiring in campaign mode. A retired base can be loaded as an enemy. It required a lot of code gymnastics, but we can finally have Keeper vs Keeper battles. :) I'll need to hook it up to the map sharing service, and implement some kind of rating or comment system, so that you don't have to download insanely hard bases or ones with dicks carved out from the mountain. :) In the future I'd like to tie this to the accounts on the homepage, so you could see how your retired bases fare against attacking players.
I also experimented with areas of influence that I described last week, and they morphed into a simple progression system of the campaign. At any given moment, interaction is limited to the three closest enemy sites. If there are ties, then all tied sites are included. Any allied sites that happen to be inside that circle are also included. The highlighted sites are ones that you can interact with. Red - enemies and green - allies.
As you conquer your enemies, the interaction circle expands. It's your job to make good choices on where to embark and who to attack and in what order. I hope that it will add a nice strategic layer to the game. At this point the biggest building blocks of this update are placed, and I'm left with implementing or fixing a lot of little details to make all of this work nice and smoothly. I estimate the update to roll out at the end of April.