Order
1. Naga (Naga Courtyard)
2. Lich (Lich Tower)
3. Minotaur (Labyrinth in the swamp)
4. Hydra (Lair in the fire swamp)
5. Snow Queen (Glass Palace)
6. Ur-Ghast (Dark Tower)
Afterwards the Highlands are yours.
Locks: swamp = hunger (until Lich), fire swamp = burning (until Minotaur), frostlands = freezing (until Hydra), dark forest = blindness (until Snow Queen), highlands = acid rain (until Ur-Ghast).
Structures are indestructible until their guardian falls. Without the previous boss a ward pushes you back.