The Fifth Realm Book by Michael Chatfield

After taking and defending Vuzgal, Erik and Rugrat have a new task to complete: build a city. They have been racing through the realms at this point, but Vuzgal is a prize that they can't simply give up on. Alva is mobilized, as are their allies, to build out the new city. It is time that they solidify their gains, working on their crafts, their fighting ability and cultivation. As they expand their personal power, their gaze turns toward Vermire, to the dungeon. It's time they started to exert the strength they've built up. Alva moves in the shadows, but to what effect? The answer lies in the Fifth Realm.