Aha, the Adventure Mode FAQ has already begun! I'm also on the lookout for the HW's maps.
7. In co-operative multiplayer, why are some of the fortresses empty?
It's because of the programming that limits the total amount of enemies that are displayed on both screens. If one player's area is very busy, then the other player's area will only begin to fill up once the partner has killed some enemies, that will split up the spawn duties of the game again. At times the game has also problems to load allies that should be there according to the map. However, it's possible to save allies in need of help even if they aren't displayed on screen; of course, that's up to luck as you can only guess from the map where they are supposed to stand.
Is this all the time? I noticed some fortresses were in neutral grey with no enemies, but I assumed those would be available at a later time in the map.