I'd rather not have slots specifically set aside for defensive structures. Half the idea of limiting the number of build slots is to force the player into making a decision: should Planet X be a fortress brimming with defenses, an economic powerhouse, or something in-between?
Given the scale that we want the building slots to operate on, i.e. pretty darn special buildings only, I personally think we will need to do something else/different for defences. This is because I can't see players being too happy wiith being faced with a decision between using one of a very limited number of slots for the equivalent of Moo2's Missile bases or for a galaxy affecting wonder...
Given the philosophy behind the idea of limited build slots (that anything most player would tend to build at a large fraction of planets should be abstracted out because it's boring...) could I float a suggestion?
I'd propose that defences of the kind that are equivalent to Moo2/3's missile base, beam base, planetary shield etc. should be abstracted out into a single "Planetary Defences" number. The presence of each kind of defence or not at a planet should be determined merely by whether you've discovered that tech or not. The effectiveness of the defences would be determined by a planet's Defences number, with the discovery of new and improved planetary defences techs simply increasing the maximum value of this number.
In addition to this there are two other, independant options:
1) Orbital defence stations - IF we want them (Personally I think they'd make nice eye candy for the combat screen, but heh) these could either A) be built and designed like ships, but fixed in location at specific planets (placement method a separate issue discussed in the build queue thread) - i.e. the Moo3 method. Or B) they could also be abstracted out, based on the planetary defences number (i.e something like you get one orbital station per 100 units of defences etc..), possibly with a limited number of standard designs based on discovering appropriate techs.
2) Super dooper defences buildings - Would we want defensive buildings
(i.e. ones that use the normal building slots) as well?
I would think these should be buildings with effects on the large scale comparable with the effects of the other buildings we were thinking of, examples might be: System warp interdictors (block movement in or out of system for enemy ships); incredibly powerful scanner arrays (detect all ship movements (even cloaked?) several jumps away); planetary lightning field generators - any enemy within a huge chunk of the combat screen gets really toasted; planetary level cloaking devices (hide the planet!); Extraordinarily strong planet shields (i.e. 10 or more times the usual maximum), etc.
(Obviously, I'm also in favour of this option. Although I can see it would be nice to be able to build these everywhere, presumably they'd have a very high cost which would prevent you anyway. Also under the slots system, the effects are on a par with the special buildings you'd be giving up, so I think the choice would feel sensible to players)