Another thought:
Dwarven Mine is -1 square, Bandit's Lair -3 squares and Goblin's Nest -5 squares. Were you considering more uniqueness to each?
What about:
Dwarven Mine: 1 free mine at lvl 3
Bandit's Lair: 1 free warrior at lvl 2
Goblins Nest: 1 free chest at lvl 1
(both 2nd and 3rd lvl freebies non-active until activation of their lvl)