Well, I'm with Feycat... But when I play I look for these things in a POI... Early on I usually go with one base because trying to do two is hard.
1.) Structure should at least be concrete or brick; but preferably Steel... There is one fabled POI that is made of steel and I've used it before in A16.4.
2.) Need to have some way to kill zeds from above.
Wither that be a roof or a structure you build or even if that's the floor(s) in your current structure... IE: You can take out sections of a floor at choke points and shoot down at the zeds. Or conversely you can take out an entire floor or most of it and accomplish the same thing. I did this with the fire station (garage area I placed flagstone and upgraded them to cobble stone and also put wood frames down) and the hospital stairways and 'entry hall' successfully. In those instances I usually line the walls with iron bars and then wood frames to ensure that they can last a while.
- the entry ways into the garage area of the fire station are choke points; lined with barbed wire and spike traps. I added flagstone upgraded to cobble stone pillars and supports for the '2nd floor' I made inside the garage area. This provided me a way to shoot down at them and also protection from vultures since I had the roof of the poi above me in the garage area.3.) Need some way to shoot at them from 'the outside' via wood frames on the outside of the building. Especially if you line the building with traps on the outside.
At times I'll line a building with 3 to 4 rows of spikes, then one row of barbed wire on top of the spikes. This is going out from the building! Then one row of blocks which are expendable. On top of those blocks I'll put barbed wire and then spikes and put barbed wire on top of those spikes and continue outwards with spikes and do that all over again. So at 'the walls' there is a major choke point!
Like below: b=barbed wire; x=spike; W=wall; B=Building
bbb bbb B
xxxxWxxxxxWxxxxB
So in the above from the zeds POV - they see 3 rows of just spikes, then one row with barbed wire on top of the spikes, then a wall with barbed wire on top of that and then the next row is barbed wire on top of another set of spikes and so on... The barbed wire slows them down before the wall, while on top of the wall and just after the wall. But as Feycat mentions - you can have a repair way in the center where there are no spikes. This will give you a way to repair all of this. Modified:
bbb bbb B
xxxxWxxRxxWxxRxB
Where R=free row without spikes
One thing I always do early on is to try to find as many towns/cities near the traitor. I also try to find multiple traitors, more traitors you have access to means you can make more money and gives you a 'better chance' at being able to buy the tools you need early! At times I'll trade off where my base is to be by multiple traitors. On top of everything else - for me this would be a 'transition period' where I'd look for another POI to put my new base at, but I would gather the resources for the base but not actually build it yet since there probably isn't enough time to get everything done... But maybe in your case - you may want to continue with this base since it maybe much easier to upgrade to concrete...