Not sure anyone falls in love with Nagasaki at first sight however it has a lot to offer.
First, it is on the seaside, and it is surrounded with hills which makes it an attractive spot for me at least. the city lives out of tourism mostly thanks to cruise ships coming every day from Mainland China with 1000’s of visitors. Mitsubishi has a massive construction site there too and that drives the business.
I found a pretty city with the Glover Gardens, the tiny streets, enjoying castella. It has a special feeling as in the history, anything that was not Japanese was called Dutch. therefore you have a lot of Holland this and Holland that but there is a Dutch connection and you may visit Huis Ten Bosch or have a look at some of the flemish style houses in the historical center. During the isolation period for Japan, Nagasaki was the only place still open to the world. This is why the castella cake has an inspiration from Portugal for example.
Because of the Chinese influence, the typical meal in Nagasaki Shippoku looks alike a Chinese dinner with plenty of food or a rolling table. But they also have Turoku Raisu (Turkish rice), Castella, oysters, seafood, champon, or Nagasaki beef.
I will go again as many of the sites are in distance from the city and you cannot walk around but need bus or even train