From The Heroes of Might and Magic III wiki

Blacksmith

Allows heroes to purchase one of three War Machines.

Fortress, Rampart and Necropolis 

Can purchase a First Aid tent for the cost of 750 Gold.

Inferno, Stronghold and Tower 

Can purchace a Ammo cart for the cost of 1000 Gold.

Castle, Dungeon and Conflux 

Can purchase a Ballista for the cost of 2500 Gold.

Note: The Stronghold also has access to the Ballista Yard, where its heroes can purchase Ballistas.

Requires: Village Hall

Cost: 1000 Gold and 5 Wood

Unusual fact: Necropolis heroes cannot normally learn the First Aid skill but their Blacksmith produces First Aid Tents.