Felltower's castle and dungeons aren't of uniform styles of construction.
Felltower Castle - built as a modern defensive structure for a highly magical battlefield. The walls are rubble-filled worked stone coated with anti-magical coating. Sections - especially internally - the walls are flecked with meteoric iron and coated with anti-magical coatings. The rubble fill is a mix of earth, chunks of worked and unworked stone, and anything else left over from construction (mostly wood bits) and compacted down. The castle was built during the era of Sterick's Barony, much like Stericksburg (which shares similar, albeit weaker, defensive structures.)
Some of the foundations are much, much older, although there are few - the very foundation stones of what was there before were largely removed or destroyed.
Levels 1 and 2 - shaped by magic and then hand-worked to finish. Sections of what the PCs have called sub-levels off of level 2 - the crypts, the funereal niches, the unfinished level that connects to Sterick's Tomb, the Prison level, etc. - are of the same construction.
Level 2 featured the first black hemispheres.
The workmanship is good, but plain, and oddly lacks any supporting arches. The ceilings are high but nearly squared off. They hold up despite lacking basic supporting architectural features like arches and support pillars. Several large rooms exist that don't have enough ceiling support yet are fully intact despite earthquakes. Many of the rooms and hallways feature art or frescoes, many of which are damaged beyond recognition (and no, Repair won't help.)
The "apartment complexes" were of the same construction but generally much more finely done and decorated.
Some of the structures, however, don't match anything around them. The "green zombie" room has a corridor leading to it unlike anything around it, and the room is equally unusual. The now-missing twinned temple was of no matching construction or design, and didn't seem to be built so much as it just was. The altar, the pool room, and a few others don't match the surrounding designs.
Sterick's Tomb - sometimes called the "dragon entrance" or the "caverns," the area of Sterick's tomb isn't constructed, it was formed naturally and then expanded with Shape Earth (some), Earth to Air (a lot) and natural and supernatural work. Its internal, full-formed and worked stone was clearly made by highly skilled workmen plus magic.
The Apartment Level - sometimes called the "Gate Level" or "level 4," the corridors are uniform and different from the upper levels. The corridors are 12' tall, 10' wide, and feature a supporting keystone arch every 10 feet. The walls are somewhat cruder and rougher than the upper levels. The floors are even but also rougher, and echoes are more easily made but thanks to the arches and walls less easily pinpointed, making this a relatively noisy level.
This level featured a large number of the black crystal hemispheres and additionally flat purple disks.
Many gates are located here, along with the "prison" rooms. These gates are sometimes in unique circumstances - such as the Olympus gate in its Olympian-construction hall - or are plain pillars but seem to have been dug out, not built into a room. In other words, the rooms around the gates seem to be have been created to access an existing gate.
The Cone-Hatted Cultist Level - the level below the "Apartment Level" is identical in construction to the level above; the second GFS is identical, as far as anyone can tell, to the previous one. The construction of the rooms spotted so far are like the ones of the level above - complete with at least one black crystal hemisphere.
The caverns - these recently discovered caverns seem natural or dug without directing intelligence (or at least directing craft and art.) The same applies for some of the cavern areas on the "Apartment Level."
Mungo's carverns - like the caverns, above, these seem dug or naturally formed. Collapses from poorly chosen digging routes and/or earthquakes, and sinkholes, are common.