In a world with reanimates, alien war machines, and the occasional raider, security is of the utmost importance. The facade and portals a settlement shows to the world can go a long way to telling how friendly or desperate an area might be. Its unlikely that any settlement will have a shoot on site policy, except for the occasional criminal they have exiled.
General Ideas for Fortification
Due to the lack of coordination on behalf of the most common reanimate types, (One, Alpha, Two, and Three) simple walls of debris or uneven landscaping can slow them down enough for a warning to be issued and armed personnel to thin their numbers. Many small settlements don't go much farther than this, preferring to be able to relocate citizens quickly, over attempting to build barriers sufficient to sop they types that can leap six meters or smash reinforced concrete.
More permanent areas will usually have at least low walls of sand bags and wood to keep out humans as well as creatures, and perhaps provide elevated firing platforms. Using the remains of a nearby building or a newly erected tower are common means of keeping an eye on the surrounding area.
Those who can get their hand on old shipping containers, box cars, or semi-trailers are fond of using them as parts of the walls. Some truly paranoid communities will often set up "killer hallways". One common scheme for this is to have a pair of heavy shipping containers in an L shape, with a machine gun at the end of one leg, waiting for the people to blindly turn the corner. Another is to have the targets pass though a lightly built trailer, and simply shoot through the walls with a heavy caliber weapon after blocking the exit.
Generally speaking, most people like their entrance doors to be based on a dropping gate. Its a form of fail-safe - loosen the clasp, and gravity should take care of the rest. Sliding or conventional swinging doors can be hard to secure things are attempting to force their way in.
Megastructure arcologies are best thought of as "fortresses constructed from a sieve".
The bases of these huge buildings are giant shopping mall and entertainment complex, originally meant to help attract outside commerce. Thus ground level has dozens of doors, plate glass show windows, while further complications pop up in trying to secure the many underground access points - sewer access, subways stations, parking garages, and the loading docks.
However, the buildings were made to withstand fire, riots, and acts of terror - in addition to the normal security gratings and locks one expects on commercial ventures. National guard presence and a high population has also allowed these places to become heavily fortified. Many of the above ground entrances have been welded shut, then have old cars lifted over the concrete vehicle stopping blocks and placed in front of them. Others can have safety gates shut quickly, and are kept under armed watch at all hours. Even should the outer walls be breached, reinforced metal doors designed to contain jet fuel driven high-rise fires can corral opponents into predetermined kill zones.
The truly surprising element about NB is how seemingly unsecured it is.
While most of the other city states tend to cluster around a centralized area seeking with strong walls and barricades, this one is set up more like a series of suburbs. Each small area is easier to lock down in case of a breach, and puts fewer people at risk. Each six to ten square block area surrounded by a double chain-link fence, all but the portal areas with razor wire strung between. Towers, or simply standing on flat roofs gives excellent visibility, and its often possible to shoot right through the fence at invaders and patch it later.
Some buildings are reinforced, and given murals to stand out from the rest. These are secondary evacuation sites, for people to escape to until rescue arrives. Armed guards are usually posted at these buildings to make sure no one attempts to steal any of the emergency supplies kept within. Most of the churches also have reinforced panic rooms to protect the faithful.
No outside problems are likely to get into Vegas, thus making it quite likely inside problems can get out.
With over four battalions of soldiers and much of their equipment, situated in a desert with clear lines of sight, noting unliving or alien is likely to approach within five miles of the former resort town. Its unlikely any traveler - presidential cabinet members including - to mover over two blocks without a check point asking to see their ID cards and inquiring bout their reasons.
Unfortunately, much of this is simply rearranging deckchairs on the Titanic. The patrols are far less about crime, and a desire for the mil-gov to run a junta. Indeed, they tend to hate the busy work, but keeping the soldiers on patrol helps sideline the tensions that might otherwise flare up.