You need to have quite a few builiding materials (Clay, Stone, and Wood? Maybe 50 of each?) as well as gold and a stable lv2 blueprint. You can find all of those things by exploring, but you may need to make some room in your warehouse in order to gather these items.

Ok, you need a Stable level 2 blueprint (they are usually much cheaper on the item trades) and some building materials and coins.

Then once you've built a level 2, you can build a level three and so on, up to 5.

You can only get level five on the battlefield portal though.