From Coppersky’s Compendium, the Go-To Guide for new players (link below):
After the tutorial you have 5 slots available to build a farm, iron storage, and
food storage.
At level 3 you can add: farm, mine, food storage, forge.
At level 5 you can add: house, farm, training camp, iron storage.
At level 8 you can add: forge, iron storage, farm, mine.
At level 11 you can add: farm, training camp, house, food storage.
At level 15 you can add: farm, mine, iron storage, forge.
At level 20 you can add: house, training camp, farm, food storage.
** At level 10 you get access to the barracks. It is an advanced building that
you build over top of an existing lvl 5 building. Building over a forge is
recommended.