Of course, city is a pretty loose term here. Most cities we see at events are more like a series of fences or a maze. However, these obstacles still serve to breakup the fighting field into corridors that at least give some sense of that style of combat.
While each section of the line engages in small scale fights over each choke point, the strategic plan must involve maintaining control over area. To do this, sections need enough coordination to block of access to the "safe zone" in their backfield via control of individual corridors. Even the smallest approach being left undefended could end in disaster for the team, especially if sections no longer have a view of their allies.
The maneuver phase of battle takes on a slightly different role. Rather than simply approaching the enemy and redressing the ranks, it becomes far more of a race to capture territory quickly. Even without any knowledge of the layout, having more territory gives you space to fall back on and better chances of controlling advantageous areas. With prior experience, pushing in quickly might give your team a chance to take key sections or corridors that lead directly to the enemy's backfield.
As the layout becomes more maze-like, or entrances more spaced apart, it is easier for a team to lose coherency of command and communication. This is where reserves can prove vital to success. By staying off the frontlines, they not only provide some security against enemies making it through, they also can gain a better view of the big picture. Reserves may find an area that allows them to maintain visual contact with more than one section, letting them balance out reinforcements against spikes of attrition or faltering lines.
Strong points often reside around junctions and along uneven corridors. These can often allow a team to control how many fighters can be on the front, increasing your own frontage on attack or reducing the enemy's on defense. For example, a corridor that narrows at one end forces the front to shrink in order to fight on that end. This funnel shape allows a smaller force to control the corridor.
Junctions of several corridors (T-junctions, etc) will often force one side to fight through what is essentially a kill pocket in order to get through. Even if the enemy doesn't fall into the trap, it will deter them from pushing that area. In this case, the stalemate might allow a few to sneak off to bolster other sections. However, having a large force controlling the area left in a stalemate might be a disadvantage, allowing the enemy to hold up a lot of fighters with a small force.
Complex junctions might even span a large portion of the town, connected by the main corridors through it. In the case above, red team's position is pretty rough. To engage with the full force up top, they'll have to fight on a very wide front in two directions. Depending on the scale, vision gaps might make it very easy for blue team to take advantage. Meanwhile, blue team just needs to fend off the small side corridor to maintain a fairly strong defensive position across the whole city section. Even though they don't control the large area in the middle, they are preventing the red team from establishing a foothold on their side of the gap.
Double tapers aren't something seen at most events, but I like the idea. I may just have to get one set up at Okfest this year. Holding the middle isn't any different than a single taper, but for either side to take it and then push out is more difficult. It ends up being easier to teams to hold ground as they move towards the middle. This also means that it may be more difficult for them to maintain a fighting withdrawal.