I have a city-state, bound to protect the evil, and the souls of the inhabitants are used to power that, and no one who enters the city can leave, so the outside world supplies it with food and supplies for free. It's been this way for a thousand years, starting with a largely high medieval technology, and they've managed to build up their technology to a roughly early 20th century level. There's about 100,000 people spread over 3.1 square kilometers.
Technology-wise, gunpowder and other explosives are generally unknown or nonexistent, as are fossil fuels. They have a steam-engine train (running on thyme, or jokes aside, vegetable oil) (or electricity?), photography (with color photos produced ala Prokudin-Gorsky), massive electro-mechanical computers for bureaucracy more than mathematics (complete police records are on a steel punched tape, and citizenship records are all on punched cards), and air conditioning for the hellish summers. Telephones and telegraphs are in limited use, due to the size of the city and efficient mail service. Automobiles are not in practical use, as the city wasn't designed for them and fuels aren't cheap. The city magic is spherical, and it stops egress by invisible wall against living matter; ballooning was briefly popular, with a number of fatalities, and a working airplane would be suicidal.
What other technologies would necessarily be available, that I would need to take into account or explain why they are nonexistent? I'm not asking about magic or magitech; there is magic, but not at an industrial scale.