The castle is built on an ages old meteorite impact location. Some theories suggest that certain emanations from the leftover material cause strange behavior in a few creatures.
In any case, we found it to be a rich trove of minerals, including jewels of high value.
The digging was accomplished by special purpose laborers and locally designed and manufactured boring machines. (Boring machines -- the only one who gets excited about them is Dead Corpse.)
Oddly, metals refined from some of these ores, which went into the tunnels, doorways, and underground ways appear to have a kind of retained presence about them. I've theorized that the meteoric impact was actually a crashed and vaporized spacecraft, and that certain nano-robots may be active from time to time. Just speculation on my part, of course, but strange things do happen down there.
Aw... yer' makin' me blush. Some of the machines we used were highly specialized for each task. We had not just the native olivine to dig through, but also some various metals and other mineral rocks to contend with. We also wanted to process some of these on site to use as building materials for certain support structures and recycling systems.
So, while one "boring" machine might just grind up a material, another might actually be designed to vaporize it, distill out certain elements from the vapor, and deposit the various materials in storage containers. We used a variety of techniques including pulsed laser, a kind of ultra-sound "chisel" that cuts the same way a diamond cutter would chip carbon crystals, various explosives, and even an experimental kind of inductor that would literally cause the rock itself to heat up and go into a molten state.
While the substrate rock is certainly unique, and the tunnels give the illusion of changing on irregular levels, my alien masters I can assure you that there was no UFO that hit here.