I don't understand what are you referring to when you say "nooks behind the towers".

I'll asume that you are meaning bastions for towers, and the nook bexind the tower may be two things:

1.- The emplacement for a cannon behind the orillon.
That cannon defends the curtain wall, the opposite (on this curtain) bastion wall, and the blind spot that can form just in front of the bastion. Here's a depiction of its angle of shooting line:

5P17RV1.1_AoF_Pos1.PNG

2.-The part of the bastion that connects with the path on the curtain.
It's just there because the ramps through which the cannons and pretty much everythig that gets to to the bastions or curtains has to go through any of those ramps, that will be connected on the inner side of the corners of the curtains. To ease the travel, there's not any door.
You may think that this is foolish, because if the enemy was able to enter the citadel there would be little to defend from there. Building something to block that possibility would make the overall defense to be less effective, up to the point where it would be worse to have that there than not having it.


About the cuantity of people needed to run the citadel. I'm already assuming that there would be needed a BIG number. no less than 500 people.

Look:
A 36-pounder required a 14-man crew, comprising one chief gunner, 12 gunners, and one powder-boy.
Source: https://en.wikipedia.org/wiki/36-pounder_long_gun

Suposing we put in every position available one of these guns, the minimum population required to defend the citadel would be of 14*40 = 560 people. Not counting cooks, soldiers, officers, petty officers, messengers, scouts, stables managers, etc.

This fortification won't be found in the middle of nowhere. You will find it near a city and defending a strategic spot.