Went on their website and don't exactly understand what is there. I saw you can take a 90 minute walking tour and visit a museum. What are you actually looking at on this walk and how do you know what everything is? Do they give you some written literature to explain everything?
Usually at NPS historic sites there is a visitors map with explanations of stops, but it is not clear, with the Fort Monroe Authority involvement if that is how this park is set up. They do say you are going to see Jefferson Davis's jail cell and where Robert E. Lee living quarters when he was at the fort. Most seems to be in the casement museum and you can take a virtual tour online of that and see exactly what you will see. If you are very interested in Civil War history then this is a site to spend some time at just because of its significance - but there are a lot of other history sites to see in this general area that you might want to devote time to.
Fort Monroe was an active Army Base up until late last year, it was closed by Congress under the BRAC (Base Realignment And Closure). A lot of people went to task to keep the State of Virginia and City of Hampton from getting full control of this place and to prevent it from falling into developers hands. At the heart of the base lies an old High Wall Moated 5 sided Fort, along with the Casemate Museum. Lots of history here, dating all the way back to 1609 with Capt John Smith from the Jamestown Colony. Google search Ft. Monroe and you'll get all kinds of info on this place.