So the new/advanced buildings were just unlocked.
It looks like stronghold lvl 25 can be constructed. You will need lvl 21 for Hunter’s Lodge, lvl 23 for Alchemy Lab and lvl 25 for Hero Academy. All there buildings can be build by converting any forge, mine or farm.
Stronghold lvl 21 requirements: 2108K iron, 8d 1 h time. You will need all you iron storages maxed in order to start upgrading.