I am not qualified to answer all of your questions, but there was one that I thought I could definitely help with. Towards the end of your post you bring up guards posted atop city walls, but I can assure you that this would not have been commonplace outside of an active war-zone or a place such as the Ukrainian steppes where raids were frequent. I have read dozens of accounts from throughout the medieval period up through the Renaissance in Europe of travelers arriving at city or town gates, and they always had to shout and loudly knock to alert someone to their presence. Most of these are, of course, at night. During the day it would be much the same if it were a castle, manor, or monastery/abbey, and if it were a city the gates would be wide open during the day to allow traffic through.

EDIT: I decided to add some more after thinking on it a bit. The composition of guards could vary greatly depending on what region of (I assume) Europe you are examining. In Italy, for example, city guards would have consisted of members of town or city militias (or possibly mercenaries, which were incredibly common in Italy around this period), and would have been equipped and trained using general funds gathered through taxation of citizens and tariffs on mercantile transactions. Alternatively, I believe rich families could also donate large sums to be used to raise soldiers or mercenaries to be used by the city. In a place such as the Kremlin in Moscow (the grand brick fortress that we think of didn't come for a while, but the site was still the Tsar's headquarters), the guards would consist of the Tsar's personal retinue, sworn to his protection. In general, guards were not terribly common outside of prisons, the homes of important people, and places such as markets, where lots of goods were exposed to the public. What were common were associations of mutual protection, or guilds, along with local, community justice. This is where ideas like 'stoning' come from: vigilantism/"justice" carried out communally.

Some sources I can think of that I know mention this sort of thing:

Voices of Morebath by Eamon Duffy

Peter the Great by Robert K. Massie

A World Lit Only by Fire by William Manchester

The Ottoman Empire by Halil Inalcik