It is a small town about 20 km from Cap-Haïtien that serves as the gateway to the UNESCO World Heritage site that comprises the ruins of Sans Souci Palace and the refurbished and the largest fortification in the Americas, the Citadelle Laferriere (or Citadelle Henri Christophe.)
The area around the Palace and Citadelle is walkable. You can take a donkey or horse up the hill to the Citadelle if you'd like, the walk can be strenuous.
The Palais San Souci and and Citadelle Laferriere. Both were built by King Henri Christophe in the early 1800s (the palace was finished in 1813 and the citadel in 1820) thanks to the work of thousands of slaves. The ruins of the palace are impressive, one can imagine the sheer size and opulence. The impressiveness of Sans-Souci was part of Henri Christophe's program to demonstrate to foreigners, particularly Europeans and Americans, the power and capability of the black race.
The Citadelle was refurbished and is at the end of a tough hike up a stone path (or you can rent donkeys.) Make sure to bring water for the hike. Hiring a guide is recommended, both to learn about the place and so he can get other guides to leave you alone -- some of them will follow you trying to get you to hire them or pay them to leave. Don't pay them to leave, just be firm. The Government of Haiti recently built a ticket stand and a small market for vendors at the foot of the path to the citadel. Built by 20,000 slaves, the building is huge and impressive. The idea was to keep the area safe from French invasions and also rival Haitians (at the time, Haiti was divided into separate kingdoms in the north and the south.) Henri Christophe's son (or maybe brother) was killed in the castle when he was smoking in a powder magazine and it exploded. Henri Christophe himself committed suicide in 1820 and his supporters buried him in quicklime (stone) so others wouldn't mutilate his corpse. You may encounter some aggressive guides or vendors at the base of the trail to the castle.
There is a small fee for both.
There are a number of vendors who set up near the entrances to the Sans Souci Palace and the beginning of the hike to the Citadelle. The government has built market stalls for them as well. They sell various kinds of handicrafts like art, hats, woodwork, and more, and you can bargain with them.
Some local people sell snacks and drinks at the top of the hike to the Citadelle. There are also some cafes in town.
Guide Maurice Etienne of the Lakou Lakay cultural centre (Centre Cuturel Lakou Lakay) offers rooms for travellers, for around HTG 1000. Staying in Cap-Haitien is a good option.
Tap-taps to Cap-Haïtien run along the main street, and cost around HTG 15 (USD .45).