Are there organised tours from the DR at the moment? There used to be some that cross the border at Dajabon and take tourists to La Citadelle, but I would not think it is recommended right now, due to the increased insecurity in Haiti.
There are currently travel advisories for US citizens as well as other nationalities that state it is not a good idea to travel to Haiti. If there are organised tours they might know what they are doing, but if you are considering an independent trip, I would say - think again.
The road from the border to Cap-Haitien and the Citadelle is extremely rough - it takes about three or four hours to cover the relatively short distance, so it would be a very tiring day trip. Travel after dark is extremely risky.