Hello all
Damn... I only recently started reading the EN boards, and I found this thread 4 days ago.
In that time, I've read the entire first thread and the rest of this thread, and then I get to this last page, and I'm too late to help.
*sigh*
I had this amusing idea that as long as we were playing with epic spells, huge versions of something like Otiluke's Resilient Sphere would be amusing.
First cast one that surrounds not only the fortress, but the enemy army beyond. Now the conflict is completely shut off from the outside world (with the number of spellcasters on our side, we should have the ritual support to make the DCs easily).
Next, cast a second one that surrounds our fortress; now the enemy is unable to touch us and unable to leavel.
Now we fill the area (we shall call it the donut, as that is how it would appear in a cross-section) with water through the liberal use of pre-planted contingency spells like Gates to the Elemental plane of Water and planted items like bottles of endless water.
As long as we maintain the spheres, the Valloreans would all drown. Just to be amusing, we could have had our epic spellcasters fashion Living Vaults in the Donut zone from which they could make secret forays out to animate the drowned Valloreans, instructing them to 'play dead' until there are huge numbers of them...