I had a city chase sequence in a pbp played in this board, starting in
this page.
You need to play with two elements: scenery and enemies, and your only purpose is to break the "I attack, hit AC X, Y damage. He attacks, my AC is Z" trap.
The scenery must be interesting enough to make the scene memorable, allow the PCs to use it to their advantage (for example, trips, disarms and bull rushes are much more advntageous when the sword can fall three stories, the enemy can fall three stories, or you can delay him for half a round. The AoO can become an acceptable risk in those cases), serve as an obstacle to be overcome, and provide ways for the NPCs to break the fight, moving to a new location and breaking the monotony.
The enemies must be interesting, and intelligent enough to avoid an all out fight with all the party. Bo9S classes are already very good at it, since they can strike and move effectively. But also you should make a good use of minions: low level enemies the PCs can swat aside with a great cleave or a spell, but can delay them for a round and break the fight into smaller segments. never use the minions in one large group, however: that leads only to grinding. Instead, divide them into smallish groups (2 to 5, depending on the PC's level) and use them at the most opportune moments.
