These open up during some PvP events at random points throughout the map. Your Primarch must travel to the portal to use it. Any enemy Primarchs occupying the same side of the portal that you are on must be destroyed before you can use it. If there’re enemy armies on the other side, it won’t prevent you from entering, but you will need to kill them off if you want to come back through from the other side. Make sure to watch the time left on the portal-- you don't want to get trapped on the other side!
These allow you to quickly move your Primarchs between different castles your team owns, especially if they’re far away from each other. To make one of these portals, your castle needs to have the ‘Tower’ infrastructure building built. Then, construct the portal. You can then send Primarchs with troops through, up to a daily capped number of troops. This cap is based on the Tower’s level, so the higher level your tower, the more troops you can move around each day. Like all other infrastructure, the portal is not functional if you have enemies on the castle, or if upkeep is unpaid.