Going to begin the teardown process. It will take a couple of days, then there's testing in a "real" game. For the moment, here's the planned/existing information about the city.
Name: The Iron General's Headquarters
Theme: Minutemen HQ and Central Intelligence Unit
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"War. War always changes.
Make no mistake: we are at war. Whether it's creatures from the wastelands, supermutants, raiders, infiltrators, criminals - all of these leech off and kill people who are trying to build a life. All of these destroy civilization.
At the heart of war is Intelligence. Know thine enemy. When you know them - when you know what they're doing - then you can trick them, outmaneuver them, and beat them. You can lure them, you can trap them, you can break their will to fight. You can destroy them.
So first: we must have Intelligence. Knowledge of what they're doing and the ability to pass that knowledge on to the troops on the ground. Until recently we did well with Minutemen runners, and will continue to use them. With The Castle back in Minutemen hands, we can increase our operations, be more effective.
You recruits have all chosen to go a step beyond protecting your families and neighbors. You have chosen to come here, to protect the Commonwealth as a whole. To rebuild the Commonwealth into a civilization again. To dedicate your lives to this goal.
Thank you.
Now let's get started. First up, ramshackle construction is not going to be tolerated. We need to be healthy, our equipment needs to stay in good shape, and we need to set an example. A lot of construction is going to be steel and concrete..."
-- The Iron General, first speech upon recapturing The Castle
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Level: 3 (fully built-up)

You'll note that at L3 the city plan is very comfortably caps-positive. The "unemployed" are actually slots available for use as:
1x City Planner/Leader
1x Radio Operator
1x Ronnie Shaw
1x Artillery
I'll be testing this more fully over the next week or so.
L0: 10 people (6 settlers, Leader, Operator, Ronnie, Artillery) - try to complete the Old Guns mission here. The central compound is built.
L1: 12 people - an ILS is built into the city plan at this point (designer's choice), if you choose not to build it then you will need to continue to babysit your city. The outer walls start to go up.
L2: 14 people - more walls go up.
L3: 15 people - the walls are complete. At this point you will have 5x Industrial plots. Once the Martial Plots reach L3 you are free to convert these to other plots, or tear them down and use the settlers to build more artillery. I suggest keeping the ILS plot, the city is not self-sufficient for food.