The Lava Castle is a huge structure composed of volcanic rock, possessing a rather large cavern system with many branching pathways, obsidian pillars and thermal vents. Various cave systems lead to the center of the castle, which is a large chamber featuring a large lava pool with tall spires of rock around the edges. No flora can be found here. Plenty of Crystalline Sulfur and Lava Lizard Eggs can be found here.
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.