RangerWickett said:
Say you've got an army of 10,000 men, most of them 4th level or less. And you want to sack a fortress that has 20 mid- to high-level spellcasters and say 500 soldiers. The fortress is on a peninsula, has high ground, and is the size of a small castle, with a surrounding town. It will take the army a month to arrive.
First of all, back in the real world, the castle if its a modern (post-crusades) type has a significant chance of surviving this sort of assault even without throwing the spellcasters into the mix. A one month journey time, plus muster and such, means that the army is going to be pushing it to last through campaign season. Plus, you camp 10,000 men in a small area and thier will be serious disease and food shortage problems by the beseiging army. (Often besiegers suffered as badly as those they were seiging.) Generally speaking, a well designed castle could hold off numbers of about 20:1 and that's what you've got here.
So, understand that if we are talking realism, this is a hard problem even before we throw magic into the equation.
What kind of tactics would you use if you were the general of the army? The general is a high level warrior-type class, with mid- to high-level bodyguards, and the necessary protection to avoid a scrysassination attempt.
Harrassment. Instead of marching 10,000 men over to a castle and camping them on an indefensible plain in full view of the enemy, I'd disperse much of my forces into bands of 10-20 each and I'd give them orders to raze the enemy country side for at least 20 or so miles around, taking whatever they could, driving off or enslaving the peasantry, and burning whatever they couldn't take. The idea here is to destroy the economy of the region, and thereby make sticking around in that castle untenable in the long run, while making the distances involved and the difficulties in tracking down any significant band of my people enough to render individual sorties by Wizards problamatic except for the most high level casters. If you send some individual 7th-8th level wizards out to kill bands of 2nd-4th level fighters, sooner or later those wizards are going to fail spot checks, or going to be fatigued by the travel, or be found sleeping, or get intercepted twice in the same day, or otherwise loose initiative and then the low AC's and low hit points are going to be a big problem. Multiple archers with ready actions are very good counters to low level wizards in these conditions.
I'd leave back a reserve force of 4000-5000 of my best cavalry and archers to meet any sorte by the enemies forces, and I'd be doing my best to recruit some high level firepower to counter individual sorties by the higher level spell casters (which at this point are barring some good fortune or mistakes on thier part I can't do anything about). I would do anything I could to avoid giving battle to a massed group of high level wizards who had the ability to retreat to safety. This probably means keeping my camp mobile and sufficiently dispersed that I'm never in danger of losing a large force to a single spell. As long as the enemy spell casters don't exceed about 13th level, this might work.
What I really would need as the commander is some counter punching ability, which I'm desparately lacking according to your description. I either need friendly spell casters of level 5+, or CR equivalent rogues/scouts/rangers, or CR equivalent allied monsters (hill giants, gargoyles, griffons, dragons, whatever) in enough numbers that spell casters can't take them out with a daily alotment of spells.