Anyways, ignore yunyasha, his layout is shit, although he was right about 3 spaces being the best, if only as a counter against airships. Seems to do wonders for splitting an army as well, but that’s really more a matter of building placement rather than spacing.
Anyways, here’s what I go for. When it comes to towers, I put em three spaces away from each other, the citadel, silos and shield generators. Makes sure that splash damage from airships ain’t ever a problem. It’s the new mage proofing, except no one uses mages anymore. Everything else can just be the usual 2 spaces away from each other.
For silos, make sure you keep em far away from each other. A silo on one side, another on the opposite, etc. There ought to be a couple or more buildings between each silo, so you can make sure that they never get all taken out at once. Having webs covering them might also do some good, so do that as well.
Let’s face it, archers are going to be your more important towers, so you’ll want them covering the citadel and each other. So in yunyasha’s layout, replace the webs and the mine with archers, and then you’re golden.
I have no idea what to do with large buildings like barracks and such. I guess you ought to spread em out so that whenever attackers see em, they’ll be thinking, “Oh shit, where’s the weak point?” I dunno, for some reason, large buildings have some kind of psychological effect that makes the cit look harder to get to.
What a lot of people (myself included) like to do is put a lightning totem behind the cit so it can’t get rushed. This might have a double sword effect in making it easier for an attacker’s army to get to the cit if attacking from the top area, so having a few extra buildings on that side ought to convince em to pick a different side. This also has the disadvantage of leaving other sides thin, but you should be able to get around that with wise tower placement. Remember, each tower ought to be covered by at least one another tower… unless it’s a flame bunker. Those things are only seem to be good as meat shields, but have em covered anyway, for what it’s worth.