I went to the Citadel in 2006 around Thanksgiving. It is somewhat complicated and dangerous, but it's possible to get there without flying. You can stay at Juan Calvo hotel in Dajabon, DR - it is on the same street as the border. Find a kid at the border to drive you on a motoconcho to take a tap-tap to Cap-Haitien. When I went, the kid whisked me past Haitian customs and at a police stop down the road, it almost became a problem, but then they just let me go through, so you might want to make sure they stamp your passport in Haiti. Take a cab from the end of the tap tap line in Cap-Haitien to a hotel like Les Jardins de l'Ocean. Hostellerie du Roi Christophe or Mont Joli. Mont Joli organizes tours to the Citadel, so if you stay there, the hotel will take care of everything for you, which I recomend. Otherwise, you can take a tap-tap from downtown CH, near the Texaco station to Milot, Haiti, where the Citadel is and find it yourself.
Have fun! You're in for an adventure!