"Me start reading the thread: some cool and interesting ways to level the city".
Then I read Gargs454 post:
"Personally, if I were the lich, I'd just toss a Deck of Many Things into the city. Problem Solved."
"Me: "BWAHAHAHAHAHA!" I probably woke up the neighbours..
It's the same as putting the party in a room full of nothing but a big red button on the wall with a deathskull sign above it and the text below "Do not push". ^^
Anyways my advice is: Let the party be the ones to have the lich's army enter, by accident or choice.
Have fun! =)