The Lava Castle is a huge structure of volcanic rock possessing a rather large interior cavern, obsidian pillars, and thermal vents. Various cave systems lead to the center of the castle, which is a large chamber featuring a lava pool with tall spires of rock around its edges. The structure seems to be the breeding grounds of Lava Lizards as it is the only place their eggs can be found.
Spoiler alert: The following section contains story related material.
Although the exterior of the Lava Castle was formed naturally, a Data download found in the Alien Thermal Plant suggests that much of the interior was excavated by the Precursor Race.