Forge

The Forge is an in-game location where you can make more powerful Equipment using Equipment you don't need or use or Ingots.

You can find the Forge on the main screen:

Interface

By pressing + or - you can add or remove items up to the maximum level of Equipment until you exhaust everything from your inventory.

The central piece between - and + shows a result you will get from using the "ingredients" you added - an Equipment item and its Level. 

You can craft Equipment from Ingots, from unnecessary Equipment (that is not currently worn by Heroes) and also combine both of these.

Crafting will cost you various amounts of Mountain Coins. The more valuable the Equipment you want to craft, the more expensive it will be.