Goblins become available when you reach lvl 2 of your Barracks.
Orcs become available at lvl 3 of the Barracks.
Trolls become available at lvl 5 of the Barracks.
Wraiths become available at lvl 8 of the Barracks. Note, that they have a special ability to pass through any buildings - even the walls!
Dragons become available at lvl 10 of the Barracks.