Depth

The most common wizard tower, alien prison, or etc. is a spire (probably). SEE: Ptolus, etc. The second most is a series of caverns, such as the Underdark, or straight-up dungeons, or so forth.

Combine some of these: a single, plummeting area that dives into the terrible depths. The depth, itself, is part of the threat and eeriness. Like a mineshaft, it is a narrow and dark and isolated space. Eventually the ears pop, the sounds warp, and it becomes weird.

The deepest cave on Earth is Veryovkina, at 2212m (7257 ft, or a little over a mile) in depth. The deepest manmade bore hole is the Kola bore, 12262m (40230 ft, 7.6 miles) deep, and only about 23 cm (9 in) wide. That's about halfway through the crust, and deeper than the Marianas trench.

This reverse-wizard-tower would be more like the first than the second, and as the natural caves and underdark sprawl, the bores are straight & deep.

At that depth (12+ km) the Kola had consistent 180°C temperatures (356°F). If we extend this to a magical world, we can expect a deeper bore, and thus hotter temperatures.