With all these conditions can 200 people sustain life?
Maybe. The main concern (since you've stated that there's ample water) is going to be food. It takes about 1 acre of land to feed one person for a year, based upon the typical 'American' diet. With a more restricted diet, you might get that down to around 0.25 acres per person.
At 264x900 ft, you've got about 5.5 acres of surface area. At best, that'll sustain 22 people if you dedicate all of it to traditional farming methods (or 'flat' hydroponic gardens). That's ignoring the fact that you'll need some of that space for living quarters, facilities, water storage, and so on.
So to keep everyone fed, they'll need to innovate. For instance, by taking their hydroponics vertical in a big way. That's the only way to get the ~50 acres of agricultural space they need into a 5.5 acre footprint. So hopefully your city block has a very tall building on it, and hopefully that building lets in plenty of light. And they'd better recycle absolutely everything, like in The Martian. Or even more than absolutely everything, like in Soylent Green.
The other option is that they send periodic expeditions to the surface to scavenge for food. But I'm assuming you want a self-contained, sustainable floating city? If so, they need to get serious about agriculture, and quickly.
How much energy is needed to actually put the city block into the air?
A lot. Lets say your city block weighs about 250,000 metric tons. That's probably on the low side, but should be within an order of magnitude of the actual value.
To raise 250,000 metric tons to an altitude of 50 feet, you need around 37,338,000 kJ of energy. In itself that falls within the realm of what's feasibly attainable. But applying it (and then maintaining enough force to sustain levitation) through electromagnetic repulsion is another matter altogether.
Let's say your 250,000 metric tons are evenly distributed. Then each square foot of your levitating city has a mass of 1,050kg. To magnetically levitate 1,050kg of mass at a height of 50 feet using magnets of 12x12x3 inches, the strength of the magnets must be approximately 3,800,000,000 Gauss. The strongest electromagnets created to date don't exceed 10,000,000 Gauss. And they're extremely large, heavy, powered by a dedicated 1,200 MJ generator, and not capable of continuous operation.
So the good news is that there's no theoretical limit on how powerful a magnet can be. The bad news is that in practical terms you'll hit materials issues (your magnet will tear itself apart when you turn it on) long before you reach the required strength of electromagnet. And even if you solve those, your power requirements will be massive (1200 MJ * 380 * 237600; which is on the order of the entire annual energy consumption of the United States), and your magnets will probably have adverse effects on any living things that happen to get even remotely near them.
Interestingly, at least according to the calculator here, if you use gigantic magnets (264 feet by 900 feet by 12 inches thick), you could get away with a more attainable 550,000 Gauss. I'm not sure if that's accurate or just the calculator being wonky due to the absurd values being fed into it. And I'm also not sure if it has any mitigating effect on your power requirements whatsoever. And I haven't factored in the weight that the magnet itself would add to your floating city. But it may help with the materials and continuous operation issues. And a giant 550,000 Gauss magnet seems marginally more plausible than an array of thousands of 3.8 billion Gauss magnets.
Though I'd suggest looking at an alternative mechanism of levitation, like perhaps something laser-based. The bonus there is that while you'll still need absurd amounts of power, you only need it on the ground. With electromagnets you need to power both the ground-based magnets and the ones installed beneath your floating city. Which can be problematic, as you're not going to meet your energy needs using solar/wind, and running a transmission line (or even wireless transmission) from the ground to your city is an obvious vulnerability; all someone has to do is disrupt the flow of power for a few moments, and your city is coming down, hard.
How would you stabilize it (toooo much wind, might cause rocking)?
Assuming it's a maglev system, and assuming all the power and continuous operation problems have been solved, I'd have controllers built into my electromagnets (Arduino boards with the relevant sensors, for instance...and magnetically shielded, I suppose) that can monitor the alignment of the magnets in real-time and modulate the electromagnetic field as necessary to correct any drift.
I don't know if it's actually possible to monitor and precisely manipulate a magnetic field in that way, but if my scientists are smart enough to solve the power and related problems, I assume they can solve this problem too.
Or if some other form of levitation is being used, it depends upon the exact mechanism. For instance, with lasers you can just adjust the target points and/or the intensity of the beams in the right way to hold the city in the desired position (or even to fly it around, so long as it maintains line-of-sight with the lasers).
If some raiders came by, what types of defenses should the dwellers
use? Raiders would use rifles, pistols, molotov cocktails, and have
makeshift shields made of garbage can lids.
The raiders are bringing metallic weapons and armor near your obscenely powerful magnets? I think that part writes itself, and just leaves them with molotov cocktails.
I'd imagine that the people living in close proximity to such powerful magnets would quickly devise non-metallic equivalents of common tools and also defensive weaponry. So they'd probably respond with things like ceramic knives, 3d-printed plastic firearms, composite armor, and so on. And a good portion of their defense might come down to "outsiders have a very poor understanding of what it actually means to be in close proximity to a 500,000+ Gauss magnet". For instance, raiders wearing steel-toed boots or metallic body armor might find themselves unable to move (or even crushed by their own gear). Metallic weapons would fly out of their hands and get stuck on the ground, etc..
It might also be difficult for the raiders to get to the levitating city in the first place (you probably can't just land on it using a metallic helicopter or aircraft; even if your airframe is principally made out of aluminum). So they either have to climb upwards from the ground (I'd think the magnet would work great for anyone with a grappling hook), or else parachute in from above (though not necessarily very far above, given how quickly the magnetic field weakens). So defensive strategy would likely focus upon detecting and eliminating threats before they reach the city in the first place.
Which again makes me think of lasers. If you've got a bank of lasers powerful enough to keep your city aloft, you don't have to divert them for very long to vaporize anything you detect ascending or descending towards your city. That could make a very strong defense in itself, to the point where the question becomes "how do you defend the lasers" instead of "how do you defend the city", because nobody is going to attack the city without first taking control of the lasers and disabling the defensive subroutines.
