Willemstad is the capital and only real city on the island of Curacao. Having said that, Willemstad is actually composed of several smaller towns arranged around a huge natural harbor with a narrow inlet providing access to it. The 'cute' part of Willemstad seen here with all its multi-colored buildings is an area called Punda.
Punda is sort of the city center area with a lot of shops and restaurants as well as government offices. Connected across the channel by a pontoon bridge is the area of Otrabanda. It's not quite as picturesque but it's just as interesting to wander around in. The pontoon bridge itself is a major attraction as it swings open when ships come into or out of the harbor. Frankly, it's one of those things where once you've seen it happen one time, you really don't need to see it again.
The Rif Fort occupies a strategic location where Otrabanda meets the ocean. There's more to this fort than meets the eye. Notably, it wasn't even built until the 19th century which is past much of the interesting part of Willemstad's history. The more historic fort is across the way in Punda but it doesn't look like much. The Rif Fort is actually a Marriott property now and the relocated cruise port on Curacao carefully funnels all visiting passengers through the fort / mall.
So it's a bit touristy and not all that historically significant but the walls of the Rif still have a great view of Willemstad and the harbor entrance. Far more impressive than watching the pontoon bridge swing open is watching a huge container ship pass through the channel and dwarf the entire town. The harbor itself holds an enormous oil refinery. It is carefully tucked around the corner out of sight from central Willemstad.
Not far from the pontoon bridge in Otrabanda is the Kura Hulanda museum of slavery. It focuses on slavery in Curacao but touches on other parts of the world as dictated by the slave trade. This is an excellent museum on a very morbid subject matter.
The relatively new Queen Juliana road bridge is the more modern way to cross from Otrabanda and Punda. It can be seen here in the background and it's another good place to get a nice view of Willemstad if you don't mind seeing it from a moving car. Incidentally, if you find yourself in Willemstad with an infant it's worth mentioning that the slightly bizarre rocking motion of the pontoon bridge was two for two in putting Alaric to sleep.
We visited Willemstad several times during our stay in Curacao. On none of those occasions was a cruise ship in port. The city seemed fairly lively with just every day activity and it didn't have a lot of the over-the-top tourist hallmarks that some Caribbean ports have. It's entirely possible they all spring up at a moment's notice when a ship arrives though.