damn - sounds like a fantastic near end to your campaign. Looking back, my last long running campaign (3.5E) had a bunch of showdowns that were big at the end, but it was not until the final battle where I had a hundred+ bad guys on the battlemat that it got truly epic.
(from memory: the final showdown with a level 20 evil cleric, his level 16 right hand and their pit fiend bodyguard, about 100 low level soldiers guarding the temple, a dozen level 12 fighters that were the royal bodyguard, as well as a couple of clerics and wizards in the low teens level wise (there mainly for dispel magics/counterspelling)... oh, and a bunch of slaves that served as human/demi human shields, some of whom the players knew. Oh, and one of the first acts of the bad guys was to gate in a fiendish advanced beholder...)
(from memory: the final showdown with a level 20 evil cleric, his level 16 right hand and their pit fiend bodyguard, about 100 low level soldiers guarding the temple, a dozen level 12 fighters that were the royal bodyguard, as well as a couple of clerics and wizards in the low teens level wise (there mainly for dispel magics/counterspelling)... oh, and a bunch of slaves that served as human/demi human shields, some of whom the players knew. Oh, and one of the first acts of the bad guys was to gate in a fiendish advanced beholder...)