Portchester Castle is actually a very good scenario even though I gave it a 3.0, the average score. It was a very nice scenario to play and although you don't really feel any special interest to it, it is very playable as it has nicely balanced objectives and very comfortable playing environment. I enjoyed building a large city and I had a very nice and large castle town in the end of the scenario. Perhaps some more limited building space would have made it even more interesting.
I played the scenario twice, first on 'easy' and then on 'normal'. On both occasions I had no trouble defending the castle but neither was I able to gather too much extra resources so the economic part was well-balanced, but I don't really think those invasions were really needed there. They were way too infrequent and too easy to defeat. Another thing I didn't like was the ability to construct anything and buy/sell any resources you want. Also, more events, like fires, would have made the scenario more interesting. There weren't many events and when they did occur, I had almost already achieved my objectives.
I didn't really find much creativity in the scenario, but there were some things like fountains, cemeteries, stone fences and stone houses (?), which are quite new things and haven't been used many times. I also raised the score because the scenario started with some peasants already working.
Map Design was pretty average. There was only slight terrain texturing and the map layout wasn't very interesting. The Portchester Castle was very nice and realistic. Perhaps some more fountains, pools, stone ponds and perhaps a bridge and other eye-candy would have raised the score...
Story was the worst part of the scenario. Actually there was no story. I would have liked to have some information about the castle and its history in the story box, but there was almost nothing in it, which definately doesn't raise the score.
But still this scenario should give you some entertainment for some time if you like building and managing a big town without having to worry about fires, thefts, or some other bad events.