CASTLES
Q: Can I choose what types of forts to build?
No. Each nation has a set of predetermined fort types for different sorts of terrain, so you can only build the specific type of fort for any given terrain. In provinces with multiple terrain types (e.g. forest and mountain), it will be one or the other, probably the fort for the terrain that has a lower terrain type number in the map file.
Q: There is a command for 'Continue fort construction'. I was building a fort and the commander was killed by an invading enemy (nation or independents from an event). I took the province back, why can't I use that command?
You are out of luck. The money's gone, your enemy tore the half-built fort down to rubble. You must start over and pay the full cost for the fort again.
Q: Why can't I build enemy national units from castles I've conquered from him?
Because the developers did not want that in the game and to keep all nations unique. You can only recruit your own troops and the native population type independent units in castles. This is not going to be changed.
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UNABLE TO RECRUIT/SUMMON ANY UNITS
Q: I can't recruit any more units, they just don't appear and the same happens with summons. What's wrong?
This behavior looks like a bug, but it is not. If you have unrest 100 or more in a province, recruitment is impossible there. If you only have problems in one province, this is the likely cause. Dominions 3 also has a built-in limit for maximum number of units simultaneously in the game. If your recruited and summoned units are not appearing, you have reached that limit and no new units can appear until enough units die off first.
Q: How high is the unit limit?
9k commanders and 140k units in the game simultaneously. Once these limits are reached, no new units can appear until some die off.
Q: What causes this problem to appear since I only get it in some games?
The problem most often happens in Late Era games where R'lyeh Dreamlands and/or Ashen Empire Ermor are present, since both of these nations generate massive numbers of freespawn units. Avoid including them in games on large maps and you will not likely experience this problem. It can potentially appear in large Early Era and Middle Era games involving Pangaea whose Pan units generate Maenad freespawn units, but this is a rather unlikely occurrence. A more likely reason would be the AI recruiting massive amounts of chaff units everywhere.
Q: Are there any other solutions besides excluding Late Era R'lyeh and Ermor?
Yes.
See the Better Independents mod. It can mitigate the problem significantly in large games, especially SP games, but it is not without its issues and was originally created to address a different issue (monotonous AI chaff hordes). Read the thread and try it out, but there are no guarantees.