I was thinking about what I'd like to see in Elemental r.e. attacks on cities, and this came to me.  I bashed it together in Photoshop real quick so you could see what I had in mind.

Essentially, the City map would be your combat map as well.  Note the space between structures for your units to stand in.  Of course, the scale would be zoomed up so you could see your units better when the city was attacked, but where the attack is initiated would determine which part of the city the attacker enters.

Defenders would be set up somewhere near the attack point, either randomly or perhaps a 'pre-attack' screen would pop up for you to allocate your defenders.

I also think structures would be attackable, with hit points.  The picket fences seen in the pic above would have a rather low defense/hp, but thicker walls would have more defense and hit points.  The attacker would have to bash through the walls to enter the city, using up attacks to do so.  Also, if he wanted to destroy some structures, presumably before withdrawing from combat, he could do so, or the defender could raze structures on the way out.

Each big square would contain a number of little squares (16x16 or whatever is appropriate) for combat scale/unit movement purposes.  This same scale could be used for frontier encounters, again with the actual terrain on the map determining the batttlefield.  I'm also thinking that mounted units would take up two squares, giants 4 squares, and dragons 8 (4x2) squares in combat.  This could cause some congestion issues when fighting inside the city...

The nice thing about this is the ability to re-use the city models for combat purposes.  They are already 'coded' and in the game, so with a little rescaling you too could be fighting to defend your Temple of Essence from would be vandals!