The Last Kids on Earth and the Forbidden Fortress by Max Brallier

Picking up after Quint and Dirk’s Hero Quest, the Last Kids are happily reunited—but quickly faced with a monstrous new mission. Inside an other-dimensional fortress, the evil Thrull, alongside a vile new villain, is carrying out a sinister plan. Jack, Quint, June and Dirk must make their own plans to infiltrate the stronghold before Thrull gets any closer to completing the mysterious Tower, a structure that could ultimately spell doom for this dimension.