In order to start reinforcing you will need to start with the most important things first which are the immediate threats. Immediate threats are doorways, windows and anywhere that a zombie can just get right in. These should be seal up so good that you can't even get back out of them. The best way to do it would be using boards, screws and nails or if it’s a metal door and frame, to weld it shut. Once all of the doors and windows are blocked up you want to do a once over on the place and make sure all the passages leading upstairs are all blocked off in the same manor. You pretty much want to seal onto the upper floor of the building with only way in or out being by ladder or something from the roof or window. Once you have the building sealed off enough as to where nothing can get in you need to work on the upper defense.

The upstairs defense will include posts and protection against climbing. With posts you will want enough so that every area outside or your defensive position is covered but few enough as to where you don't need too many people to man them. Usually 2 or three posts can cover an open area if the posts are elevated and have no obstructions blocking its field of view. At first you just need a basic spot designated to stand but eventually you will want to build a roof for the posts and wind protection because if you have ever stood any post before it’s the elements that really put a hurting on the post stander. As for protection against climbing, this is a defensive measure not so much for zombies but to control travelers from trying to climb in and steal anything. Remember zombies are your main threat but desperate people can become another one. Barb wire makes good anti-climb protection by stringing it all along the sides of the building making climbing nearly impossible. Another good thing would be fixing chain-link fencing all around the top of the roof creating sort of a lip which prevents the person climbing from getting up and over it.

Just remember that a defensive position is never perfect and improvements can and should always be made and constantly inspected and improved on.