From The Heroes of Might and Magic III wiki

[edit] Cliff Nest

The Cliff Nest is the level five dwelling of the Stronghold town. It produces Rocs at a rate of 3 a week.

Requires: Fort, Goblin Barracks, Wolf Pen

Cost: 2500 Gold, 10 Ore

[edit] Upgraded Cliff Nest

The upgraded Cliff Nest allows the production of Thunderbirds

Requires: Fort, Goblin Barracks, Wolf Pen, Cliff Nest

Cost: 2000 Gold, 5 Wood and 5 Ore