The forge is the place where you discard runes that can't be placed on the board. When you throw 4 consecutive runes into the forge, the game is over. But for every rune you place, the forge cools off by one level.
You can also discard runes that have an available space. You should not hesitate to discard a rune if the only space available blocks other spaces on the board.
In the easy mode only, a red "no" circle will appear on a rune if it needs to be discarded.
In Time Trial mode, the forge fills up automatically, so be quick when placing runes on the board.