100 people is about 5m² per person. That's already a crowd. The island is smaller than Times Square. It's 100 times smaller than Vatican City, it's in fact a fifth the size of St Peter's square.
49 m² per person, enough for a small house, not 5. If you build a skyscraper (or, more ambitiously, underground) you could have luxury space for hundreds of people. Energy would be a problem, but not as much of a problem as the Belizean government.
$ units
Currency exchange rates from FloatRates (USD base) on 2019-05-31
3460 units, 109 prefixes, 109 nonlinear units
You have: 1.2 acres
You want:
Definition: 4856.2277 m^2
You have: 1.2 acres/100 people
You want:
Definition: 48.562277 m^2
Less useful in the era of high explosive artillery shells, much less high-altitude bombers, cruise missiles, ballistic missiles, and precision-guided munitions.
I think this has nothing to do with defence and everything to do with Kowloon Walled City that was sort of a no law enclave where everything was built very high and shoulder to shoulder to maximize floor space.
Oh, I was just thinking of https://en.wikipedia.org/wiki/Walled_city, which describes nearly all cities for the past ten thousand years, though less so for the last 5% of that time.