I had a hard time finding the lava castle, so I mapped my path back to share it with anyone who is having a hard time finding it. This is a very scenic path, going through some of the most interesting scenery in the game.
Before you go: You will need your Cyclops and Prawn Suit upgraded to max dive depth. Also, I highly suggest having a thermal recharger and a medpack dispenser onboard your Cyclops, because the lava larva will constantly attach to your cyclops and drain your power, so you have to go outside into the boiling water and scrape them off.
You can make it into the lava castle with just the standard Prawn jump jets, but I recommend either the grapple arm or upgraded jump jets to make life easier.
(Note: In case you don’t know, hit F1 to get your coordinates.)
Start at -554, -493, 1271. This is a good ways NNW of the starting position. Just go to that coordinate on the surface and dive straight down. There is a cave there leading down to a single blood tree at -698, -571, 1067. This is a tight squeeze, but the cyclops can make it.
Past the blood tree, there is a ghost forest cave. This is kind of hard to navigate the cyclops through, so you will have to weave in and out among the trees. You want to get to -699, -642, 570. There is a cave there directly underneath some hanging moss. You will go almost straight down through this cave.
Travel to -676, -807, 347. This is the entrance to a large chamber holding a huge egg tree. (Not sure of what it is really called, but it looks like a big egg to me.) Take screenshots for your friends back home. Travel through this cave to the other side. There is a cave leading almost straight down under a blue waterfall (-1293, -911, 421).
You should now be in the lava zone. (Inactive? Active? Who knows). Just follow the cave on a somewhat straight path to the lava castle. The actual entrance is at -60, -1190, -23. It’s a small cave on the side near the top, with alien columns on both sides. The easiest way to find it is to look for a small lava flow coming out of it.
You can take your prawn suit into the cave and just follow the path to the large chamber where the power plant is. Use your prawn’s thrusters to jump onto the big cables coming out of the side. You should see the entrance (a large port with a energy field) on the side near the top. You will need 2 purple artifacts to get to everything in the base.
I hope this helps someone. I apologize if anything is incorrect or unclear. As I said, I wrote this in reverse order. Also, I am pretty sure I came in a different way to get to the giant egg tree chamber. I know there are multiple paths.
Also, as a side note: Beneath the castle there is a huge open chambger with a large lava lake with small alien structures on top. There is also a giant ash(?) pit near there that is too deep for the cyclops. I explored it with the prawn, but I could not find anything there.
Note: Changed power cell charger to thermal charger. Power cell chargers are no longer feasible in a cyclops, but a thermal charger will keep your power cells charged in the lava area.
Original Link – Continuation of discussion