Gehennom mapping

Gehennom Mapping

Gehennom contains a large number of maze levels. Maze levels always fill the map, unlike dungeon levels, and you can only map a few squares per turn by wandering around. Digging is slow, because wands or spells of digging can only dig 1 square per-shot on a maze level.

This has given Gehennom a reputation as a slow and boring phase of the game. But the amount of work involved can be greatly reduced by good strategy.

Depending on your role & equipment, mapping can be made a lot faster, but writing and using a lot of magic-mapping scrolls uses up marker charges, which is a luxury which can't always be afforded. Since fast mapping of Gehennom is a luxury, most players will want to expend minimal resources on it - wishing up a spellbook of magic mapping is not cost-effective unless you really have wishes to burn.

The Alternate Level Method

The core of this strategy is the alternate level method. This is a way of simply halving the effort required to map Gehennom.

Observation: If you map a level, either by magic mapping, or manual exhaustive search, you find both the upstair and the downstair.

Observation: going down the downstair on level N puts you on the upstair of level N+1; going up the upstair on level N puts you on the downstair of level N-1.

So, if you map level N-1 and level N+1, you can find the stairs on level N without mapping it - go down the downstair from N-1 and you find its upstair, and going up the upstair from level N+1 gives you its downstair.

Conclusion - if you can map level N, N+2, N+4, ... through Gehennom, you get all the others for free.

Typical application of the alternate level method is: you arrive on level N+1 by the stairs, so you know where the upstair is. Step aside & dig down, falling through to level N+2. Map N+2; go to the upstair, go up, and you now know the downstair for level N+1. Go back down, go to the downstair for level N+2; go down to N+3. Rinse and repeat.

A source of digging is therefore required. I normally carry a pick-axe, so I don't kick myself later on for using up all the wands of digging. Note in particular that, because you don't fully map level N+1, you don't know the most efficient way between the stairs, so you usually need to dig a path between them; this can get expensive on digging, so a pick-axe is recommended.

Refinements

(This bit is more spoilerish)

Finding Vlad's

If the castle is at level C, you may need to map all of C+9 to C+13 in order to find the upstair leading to Vlad's (which can't be found via the alternate level method). If you aren't short of mapping, do them all as you encounter them; if you are tight for resources, do C+9, C+11, C+13 as per the method, and only go back and do the others if you haven't found the stairs for Vlad's yet.

Known levels

If you are happy to be spoiled by reading the Gazetteer, or have played through Gehennom before, you'll know the layout of the major demon lairs. So you know where the downstairs are on those 4 levels, and know roughly where the upstairs are. So you can treat those levels as already-mapped; if you find a demon lair on level N, you can restart the alt level method from there, mapping N+2, N+4 etc.

If you get lucky, you find demon lairs and alternate levels, and don't have to do any mapping at all.

Mapping Gehennom in Practice

Depends on the equipment you have.

Spell of magic mapping

If you have plenty of magic power, no need to be clever, map every level.

But most non-wizards don't regenerate magic power very fast, so have to be more economical. Use the alternate level method - it greatly reduces the number of levels you actually need to cast mapping on.

Wishing up a spellbook of magic mapping simply isn't worth it - wish for a magic marker and write Scroll of magic mapping as you need them - cheap to write.

If playing a wizard, while you can write a spellbook of magic mapping, this is normally not cost-effective; for the ~36 marker charges it uses on average, you can instead get 6 mapping scrolls, so assuming you have around two mapping scrolls already in stock (you'll usually have found a few during the game so far), you get maybe 8 mapping scrolls total which is enough for Gehennom using the alternate level method. And the cost is on-demand instead of up-front, so if you suddenly desperately need something else instead, you haven't used up the option; and mapping scrolls can be squeezed out of almost-exhausted markers, whereas magic mapping spellbook needs a marker with 50 charges in to be safe.

Scrolls of magic mapping

This is the main strategy. Most players will have got a magic marker, by finding or wishing; and most will have found 2-3 mapping scrolls by chance earlier in the game.

Use the alternate level method; to map, use any scrolls of mapping you have first, then write more if you have charges to spare and some blank scrolls. Usually by this late in the game you have already done any armor/weapon enchanting, got all the stuff you need, so you don't typically have anything to save marker charges for (except perhaps for portal detection - don't forget).