Capping the game at 12th level
You're welcome Narfellus,
Since you're comfortable at capping your campaign around 12th level, you can start by looking at what 12th level is by looking at the maximums:
1. 88,000 gp starting wealth (the DMG suggests that no single item s/b more than 1/4th the value of the character level starting wealth so that comes out to 22,000 gp). We know that 22k gp buys:
a.+3 weapon (maybe a material component thrown in)
b. +4 armor or shield with some custom stuff on it
c. +3 amulet of natural armor or ring of protection +3
d. +4 ability enhancing item
e. +4 cloak of resistance, and so on...
In my campaign, I capped the player characters at 1/4th the starting wealth of any single item they can buy, so if they go into a mageware shop, they will find only items of 22,000 gp or less for sale. This helps me keep balance, and I would never give out more treasure than what they characters can buy (I wouldn't throw in a +5 sword at a 12th level adventure).
2. At 12th level, you know that 6th level spells for the core spell-casting classes are the max, so you got your disintegrate, anti-magic, and chain lightning as your heavy hitters. To make encounters more balance, but if you want to throw in monsters that are designed for higher levels, cap out their spell-like abilities at 12th level if normally higher, deny the monsters access to their 7th level and higher spells and spell-like abilities, but to keep the same "punch", you can opt to keep the same hit dice. This makes combat a little nasty, but it doesn't guarantee a TPK everytime. For example on the balor example being thrown around, he's a baddie at CR 20. I would take away his +1 vorpal longsword (since it's a +6 weapon), and reduce it to a +3 weapon of some type. Throw away the spell-like abilities that are higher than 6th level spells (goodbye fire storm, implosion, power word stun, insanity, and greater teleport), and reduce his caster level from 20th to 12th. If you keep his HD the same, he'll still be a nasty opponent even for a party of 12th level characters and you haven't had to do a lot of work in making him a more appropriate encounter.
By doing this, you've managed the height maximums of your campaign setting ahead of time (if you envision the balor as the ultimate monster for the party to defeat--if you choose another monster like an ancient great red wyrm, it'll take more effort to work down if their original CR is WAY higher than 12).
I don't own any of the low-magic campaign settings, but I see in the threads that some of the game companies have approached that design and some work. That's great if they've balanced mechanics and a believable setting.
A good DM tip is that if some of you want to run higher-level campaigns, but it's too much of a headache to keep track of, just work out these details:
1. Religiously follow the wealth table on page 135 and set a maximum value of any character buying or making an item at 1/4 the starting value, so if you have a party of 12th level characters, then the maximum value of any single item they own will be no more than 22k gp. This takes out a lot of headaches, because then it can serve as a guide for treasure to give out in adventures and so on by never giving out anything more in value for the characters to have.
2. Know your spells. If you want to design an adventure for a higher level party (say 15th level), think of the maximums as I've described above. 15th level is 8th level spell casting for clerics and wizards, what can they do? Magic items are just going to emolate a good deal of spells and if you keep the cap, you don't have to worry about characters getting crazy when they purchase magic items or make their own.
3. Know your outside d20 sources. If you're the type that easily allows anything in the PH and DMG, but nothing else, that's usually enough, but outside d20 stuff may have "game-breakers" in the form of magic items, spells, character classes, prestige classes, etc. Some of this stuff will have inherent flaws in their design in that if you allow it, you only discover to your horror that some player has discovered the amazing edge it gives him in every situation. Not very fun to ban it after game play. I'm a big fan of d20 stuff, but I know that some stuff may not be appropriate for my campaign, and I have to review it all ahead of time before I can allow players to pick the stuff up.
