Modular High-Level Modules

As a complimentary post to the one on low-level modules, I thought I might dwell on the idea of making modular modules for deeper level dungeons. As JDJarvis noted, higher-level modules are often hard to fit into existing campaigns, because they often include personalities or details that violate what players have already discovered about their existing campaign setting. Plus, the access points on the surface for such adventure locations either include whole new settlements that might not fit into the campaign setting themselves, or they have simple access points that must be located far away from where the players have already traveled, in order to explain why they haven't noticed the location already.

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.