Hi The_Stranger,
I missed seeing this post where you ask how defenders and buyable units work on miscellanious structures. In case you may still need to know I will try to explain.
As you know there are basically three sets of indexes for defenders and the units within which are structured like this:
0-9 (easy)
10-19 (average)
20-29 (hard)
Now all recruitment sites don't work exactly the same unfortunately, so let's start with how the circus and rogue shack work.
When you fight the guards from any of the three indexes, the buyable units within will be from the same index group. Now this doesn't mean that if you fight the guards in say index four, the buyable units will also be from index four. They may or may not be, it's random. The buyable units will simply be from the same difficulty level of indexes as the guards were.
Now as to Critter Dens and Druid Circles, according to all tests I have run these simply do not work properly, at least on random maps. On set maps they can be set to contain units within the easy, average or hard indexes, but on random maps only those units in the average indexes ever appear. The guards from all three groups of indexes come up but not the units within. As to why this is, I simply don't know. My guess is that it's just a bug.
Anyway, this is simply stuff I discovered as I worked on my own mod (which is now available by the way ). Hope it helps.Thanks.
Kirky