Stage numbers:
100 - Gransys
200 - Cassardis
210 - The Encampment
220 - Gran Soren (normal variant)
230 - Gran Soren (everfall variant)
240 - Gran Soren castle
250 - Cave inhabited by ancient arisen
300 - Witchwood
310 - The Greatwall: First Floor
320 - Large underground/basement area. Catacombs?
330 - The Shadow Fort
370 - The Bluemoon Tower
380 - The Tainted Mountain Temple (also used for prologue)

(all 400 stages are Bitterblack Isle areas)
400 - Entrance
401 - Shortcut linked to entrance
402 - Second shortcut linked to entrance
405 - Safe area with healing water
406 - Safe area again but with no healing water?
410
413 - Large city area
420 - Stage with cockatrice?
421
423
424
425
430
431
435
436
440 - Circular arena where you fight the gazer?
443 - Rotunda of Dread?
444 - The Black Abbey?
445 - Sparyard of Scant Mercy?
446 - Circular area where you fight armoured cyclops
447 - Variant of 446
450 - Area where you fight final boss

500 - The Tainted Mountain (first part of Dragon boss fight)
502 - Second part of Dragon boss fight
600 - Everfall
601 - Everfall (late game variant)
602 - Everfall dungeon
603 - Everfall dungeon
604 - Everfall circular arena
605 - Ur-Dragon arena
606 - Unused stage
607 - Unused stage
608 - Unused stage
609 - Unused stage
700 - Well in Cassardis
701 - The Watergod's Altar
702 - Ancient Quarry?
703 - Unused stage
704 - Soulflayer Canyon?
705 - Some kind of dungeon with water? The Blighted Manse?
706 - Cave underneath the Shadow Fort?