With only a couple of hours of daylight there wasn't a whole lot of time to explore but at least I saw more than I did last time. That's the thing about these work trips. We spend so much time working that we never get the chance to check out the sights. Normally it's just enough time to grab dinner, check e-mail and get to bed before the next day starts.
Portchester Castle is at the north end of Portsmouth Harbor. Originally named Portus Adurni, it was built in the late 3rd century and it is the best preserved Roman Saxon "shore fort". Henry I turned it in to a royal castle in the 12th century. It was enlarged in the 14th century under Richard II and this is where Henry V assembled his troops for the battles against France.
Portsmouth is also home to Spinnaker Tower. I'm sure the view of the South Coast is great at the top of the 170 meter (~558 feet) tower. Maybe next time.