This is about Kingdom: Two Crowns
The iron technology or Iron Age is the technological tier unlocked at the iron mine, that, once paid twenty coins, gives access to:
Builders are not required to build the iron mine as it builds itself.
Without this tech, the Kingdom structures can only reach the stone tier. After paying for this tech in the fourth island, the Monarch is allowed to upgrade them to the iron tier on any island, even if the Monarch loses the Crown.