There are two distinct modes of fixed location survival retreats: ”In Town” and “Isolated.” The former depends on some local infrastructure while the latter is designed to be almost entirely self-sufficient and self-contained. Isolated retreats are also often termed “remote” retreats.
Not everyone is suited to tackling the tasks required for self-sufficiency. Advanced age, physical handicaps, lack of trustworthy family or friends, or chronic health conditions could rule that out. If that is your situation, then you will probably want to establish an inconspicuous “in town” retreat rather than an isolated “stronghold” retreat.
If opting for “in town,” buy a masonry house with a fireproof roof on an oversize lot. (Make that wood frame construction if you live in earthquake country.) Carefully select a town with a small population—somewhere between 1,000 and 3,000 if it has a true “end to end” gravity fed water supply, or from 200 to 1,000 if the water system is in any way dependent on the power grid. (The 1,000 upper limit is for fear of sanitation problems.) IMO, towns and larger than 3,000 lack a cohesive sense of “our community”, and any town with a population smaller than 200 would lack a sufficient mix of skills and the manpower required to mount a sufficient defense in the event of a true “worst case.” I believe that it is best to avoid larger towns. At some point over the 3,000 inhabitant threshold, the “we/they paradigm” will be lacking, and in a true TEOTWAWKI it could be every man for himself.