In the Troll Empire you can do pretty much everything you can in the human empire, execpt kill trolls. You start out at level one, but your leveling will be the same as it was for your Human Empire so you don't have to worry. To get it you have to rise to level 30 and buy it with 200,000 gold or 22 cash. In the Troll Empire you can get the Barracks, Stables, Archery and upgrade it like in the Human Empire. Also, you can have Troll Villagers (but they're called Mr. or Mrs. Troll).
You can get every troll in the unit index except for the Rehabbed and Nice trolls.