One thing I suggest is that a high-level module could be designed as a simple level or sub-level, with the intention of adding it to an existing dungeon. If you need to explain why this level was inaccessible previously, or if you want a direct access route from the surface with limited accessibility based on character level, you can make access routes depend on the use of spells or magic items of higher level. You can use spell effects of (dungeon level / 2) to limit access. Here are some suggestions:
- dungeon levels 4-5: secret door with opening mechanism located in middle of high ceiling, accessible by Levitation, or with an invisible mechanism located just above the character's reach, locatable with Detect Invisible or Locate Object; Wizard-Locked iron door.
- dungeon levels 6-7: dispel-able magic barrier; opening mechanism deep under water.
- dungeon levels 8-9: wide, deadly region by-passable with Dimension Door; opening mechanism that opens when touched by a specific monster type; cursed portal.
- dungeon levels 10-12: thick rock walls block access route.
- dungeon levels 13+: thick iron barrier blocks access route.