What do Shelters do?
When upgrading Shelters, the upgrade bonus alternates between increased training speed and increased soldier training volume. Even numbered level upgrades (ex: Lvl 3 to Lvl 4) increase training speed, while odd numbered level upgrades (ex: Lvl 4 to Lvl 5) increase soldier training volume.
Shelters can only be built in empty building locations outside of the Base Wall. Click on an empty spot and choose the Shelter to build it.