What is the best time to attack the drow city?

I want to do as many quests in the city as possible but i also want to attack them
my group is around lvl 18-20 and i think i can take them on quite well

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I had Viconia cast Protection From Evil 15', then Gate near the large group at the center of town. Then, it's just a race to see who can kill the drow first, you or the pit fiend.
Good PC: They are evil, let's kill them all!
Neutral PC: They might go after me and try to avenge the few ones I killed, let's suprise them and kill them all!
Evil PC : Lets kill them!