Here are some tidbits I learned about megadungeons & GURPS that I've been meaning to set down somewhere but which don't quite fit anywhere just yet.
- Long straight hallways make exploration easier for people with access to Dark Vision. They can see all the way down them. Liberal use of doors, turns, and vision-obscuring devices like curtains, smoke, and temporary barricades will help spare you, the GM, from describing the entire map from end to end if someone situates themselves well and has this ability.
- Wizard Eye makes that go double - now turns and twists won't do much for you, either.
- You really need to know the DR and HP of everything. Be prepared for occasional argument about how it can't possibly take that much damage to knock though 10' of stone based on how this one time one player knocked down a brick wall during house repairs.
- Wandering monsters are a tax on the GM, too, and may only cause more rest and more inclination to the realistically-reasonable 15-minute workday. Keep other hazards on hand for resting too much, like monsters that get more prepared or general hazards (smoke, lack of air, temperature) that make taking your time bad.
- PC groups, especially in GURPS, will likely favor open-order formations. These can be long. A group of 10 PCs and NPCs will likely take up a minimum of 5 yards depth, and likely more like 9 yards if they array 2 across a 3-yard corridor with gaps for second-rankers to fill in for a combat space. This will affect traps, combats, and spell effects.
- GURPS PCs can generally spam out a lot of lower-power magic but less high-powered magic. Couple this with an emphasis on defense, healing, and buffing, and combats with large groups will go slower even if the PCs have the upper hand thanks to superior fighting power. This will eat up big chunks of sessions and reduce the overall pace of exploration.