A few quick notes, die_kluge.
One, make sure you examine Piratecat's Story Hour, My Story Hour and Sepulchrave's Story Hour for some examples of ways to challenge high level players and keep them on their toes. It can be done, and done well. At 13th level, you've cross the threshold into mid-level play...in a couple of levels, you'll see what true high-level D&D PCs are capable of, and it's scary.
General Quick Notes:
1.) Time constraints. It's been mentioned before and I'll reiterate it: PCs who are not worried about some form of deadline are less threatened and better prepared. When they retreat to rest, make the world change in their absence. Pressure them some what, but don't crush them. Have the castle become abandoned, or the defenders restock the defenses. Making custom items takes time...force them to make hard choices.
2) Constrain rewards. The higher level you get, the harder it should be to find the exact kind fo treasure they want. Leave a +4 pickaxe instead of that longsword they need. Force them to hunt down and find (or quest to create) the items they desire for their characters.
3) Be aware: the CR system is less reliable at high levels for a reason: PCs power-bases become so divergent, it's hard to make a single designator for such divergent collections of abilities. A group of 18th level characters with no arcane caster but lots of different weapons will have a different experience than a collection of arcane and divine casters against a mithral golem, for example.
4) Be aware that the CR system assumes 4 characters and certain conditions. You have
7 players. Encounters need to be planned differently for them. Adding additional monsters of the same type isn't going to be the only solution. Despite what the DMG says, the equation is not so cut and dried. With this many players, most of them aren't going to be doing much per combat. Spell resources aren't going to be as precious, and folks may even clamor to do something.
5) Remember the rule: "Everybody rolls a 1." Even the most powerful monster may fail a save...will, in fact, do so eventually. Plan accordingly. The same applies to players, for that matter.
6) Mix up the environment: have the players battle on other planes, underwater, in the air and on dangerous terrain. have them fight in a snowstorm or a thick fog. Challenge their expectations and perceptions.
7) Remember that CR alone doesn't tell the tale because it assumes a
series of encoutners. A party should have little trouble dispatching a monster of the appropriate CR. The issue is the draining of resources. "Damn, I should have saved that dismissal!" If the party thinks that more battles are coming soon, they'll be much more likely to hold their resources back, making combats more challenging. Conversely, a suprise attack when the group isn't ready, particulary if the casters are prepped for something different altogether, makes what might have been a simple battle much harder.
8) Remember to let them win and strut their stuff. They earned the powers fair and square...let them enjoy them.
9) Bend the rules. Send a unique creature at them, such a unique demon with DR 15/silver or DR 10/crushing, for example. Heck, make it DR 20/Cheese, for all they know. Change a creature's appearance and confuse them.
High-level play can be fun, challenging and every bit as viable as low-level play...but it is a different game, and needs to be addressed as such.
If you want more advice, I'd be glad to give it.
!
