The problem is to protect your survivor population both during the initial outbreak, and provide security for long term survival. You also must plan for sustainable supply of resources. Assume that you can both identify survivors, and that you have sufficient resources to protect survivors during the initial hot contagion period (food, medicine, ammunition, et al). And assume that your leaders have both the foresight and the political willpower to enforce the necessary changes to preserve the citizens.
Initial protection period could be bootstrapped using existing police and soldiers, firefighters and construction crews. Assuming you have access to sufficient resources, you would assemble emergency barriers, choosing strategic locations to erect temporary barriers, leveraging existing buildings to provide part of the natural fortifications. The erection of barriers would proceed simultaneous with demolition of buildings and obstacles outside the barrier to enforce a kill-zone 200-1000M wide around the safe zone (how fast do your infected move?).
Since the threat is low-tech (infected, low speed), you would likely need to erect a barrier between 12-20 feet high. Temporary fencing would suffice for some areas (short term), but would require constant vigilance from defenders. Chain link is too easy for infected to scale. You need entrapping obstacles in the kill-zone. As the initial situation stabilized, you would continue to reinforce and improve existing barriers and erect new barriers.
You could quickly erect walls around parts of the city using existing semi-trailers and train boxcars. These obstacles could be installed as quickly as the survivors could drive or haul them into position. This initial barricade may only be 8-10 feet high, but augmenting the barriers with a 3-4 feet high battlement would protect the survivors from contact with infected.
Once the initial onslaught of infected has been resisted or repelled, ongoing fortifications could be constructed by trenching and obstacles in front of the walls, and as time and resources permits, combining demolished rubble with concrete to pour more effective walls. Remember, the threat is from infected, who have been reduced to primitive weapons (no guns or munitions). As the walls continue to improve, the trailers and boxcars can be dismantled to provide metal sheathing for the walls (again, no explosives being employed by threat).
Your initial city may have irregular shape, due to exigencies during initial fortification construction. During this stabilization period you would correct the perimeter to the best shape, especially as the barriers grew more effective. Guard towers would be erected to provide enfilading fire from height (again, no snipers amongst the infected). And you would also have no threat of engineers or sappers from the threat side to tunnel under your walls.
All of the classic problems with walls - explosives, siege weapons, munitions, sappers, trenchers, engineers - pose minimal challenge. The infected are a threat that does not pose serious problems to the classic weaknesses (other than needing distance and firepower). Walls need not address the myriad weaknesses that normal external threats pose. Think about the siege of Stalingrad as an example here.
Assume that the food, medicine, and ammunition challenges have been managed to this point, then you need to ensure that you have sustainable food and water supply, sewage, and industry. Cities are often located near water and sewage, and we could assume that a nearby power generation (hydroelectric, nuclear, geothermal, or solar) could supply sufficient energy for the security lighting, and reconstruction efforts.
Food is the problem. You cannot grow enough food inside the city walls to feed the population. The land is likely not sufficiently arable, the livestock was not kept near the city. You have too many people for the land, and you need enough people to guard the walls 24x7, and provide all the industry you need to sustain the city. You could assume enough recycling efforts to provide materials, and your city might be located near a quarry or mines, providing ample natural materials close at hand.
What you really need are natural barriers. You need barriers provided by harsh or impossible terrain. Think the cliffs of Dover for example. Typical cities are located on rivers, at ports, with very little natural terrain providing defenses. Other than buildings there is little to impede the leisurely stroll to approach many modern cities. But San Francisco, Manhattan, or Long Island have limited access other than by water. Can you identify a city that provides natural barriers on two or three sides? Could you devise a way to make the water a natural defense to the infected?
Identify those cities which have substantial natural barriers, and your survivor cities would only need a fraction of the defenders that a city requiring a complete wall would require. Perhaps your city is located on a natural rise, and you can engage earthmoving equipment to cut a 20-50 foot high wall (faced with metal sheathing) around large parts of the perimeter. Perhaps the infected cannot swim, and water prevents their approach.
