The Great Forge is a building.
upgrade => Blast Furnace
This building is an improvement of the Forge. It therefore requires the latter to be built.
The creation of the Great Forge is only possible if the technology Iron Tools has been enabled in the province.
This building requires 9300 man-days
The construction of the building requires 2 political points, 7,000 gold, 10 wood, 10 iron and 20 tools.
This buildings allows you to stock more iron (200 instead of 100) and tools (200 instead of 50).
It improves the productivity of the manufacturing of tools.
Experimental data (from French section): The production time of tools is decreased by 24%. For 10 tools, it goes from 984 to 752 minutes per worker.