Forges are buildings that can be placed on field tiles after the Smithery technology has been researched. It gives +2 population to the city it is built in for every adjacent Mine, no matter if it was built before or after the Forge.

It costs 5 stars to build a Forge, and each city may only contain one Forge.

Forges can also be built next to enemy mines, which lowers your population. Remember this when choosing a location for your forges.
Forge Lvl 2

A level 2 forge.