The Roman walls
Third-century fort walls standing near full height with their D-shaped bastions, and you walk them for free.
The walls that enclose you here are Roman, not medieval. They went up in the late third century as a shore fort, most likely the Portus Adurni of the Roman lists, and they still stand close to their original height, D-shaped bastions and all, which is why this is routinely called the best-preserved Roman fort north of the Alps. That alone sets it apart from every other English castle: a vast stone square more than 1,700 years old, open to the water of Portsmouth Harbour on its southern side, with families spread out on picnic blankets and children running across a nine-acre green that costs nothing to enter. What you pay for is the inner bailey in the north-west corner: a square Norman keep begun around 1100 and raised in the mid-twelfth century to roughly 30 metres, and its roof is the reason to climb the narrow spiral stair. From the top the whole harbour lies below you, out to the Solent and, on a clear day, the Isle of Wight. The keep floors hold a compact, well-made exhibition on two later chapters: Richard II's palace of the 1390s, and the Napoleonic Wars, when the castle became a prison camp for more than 7,000 French captives, among them around 2,000 African-Caribbean soldiers taken on St Lucia in 1796. Their story is the part that stays with you. In the south-east corner of the bailey stands the Norman church of St Mary, founded as a priory in 1128 and still in use. Practically: a train from Portsmouth Harbour or Portsmouth & Southsea to Portchester takes about 10 to 15 minutes, then it is a mile on foot (around 20 minutes) through the village to the gate. Driving takes 20 to 25 minutes from central Portsmouth via the M27 (junction 11) or the A27; the pay car park is 200 metres from the entrance and holds the only toilets on site. An adult keep ticket at the gate is £10 to £12.70 depending on the day (off-peak weekdays are cheapest, weekends more, peak summer dates most), with 15% off for booking online by midnight the day before, and a family ticket for two adults and three children runs £25 to £32. Advance booking is not compulsory, but the discount makes it worth doing. The honest verdict: this is a half-day trip, 90 minutes to two and a half hours including the walk from the station, and it earns that time if Roman walls and the view from the keep appeal to you. With small children the pushchair works for the grounds only; the keep is for steady legs, its stairs steep and slippery in the wet. Most importantly, the keep and exhibition close from early November to mid-March, leaving only the open bailey in winter. It remains a fine harbourside walk, but with no roof and no real cafe (in season the shop sells only snacks and hot drinks), so save a winter visit for a dry day.
Third-century fort walls standing near full height with their D-shaped bastions, and you walk them for free.
Some 30 metres of narrow spiral stair end in a view over the whole of Portsmouth Harbour to the Solent.
The story of more than 7,000 French captives, including some 2,000 Caribbean soldiers from St Lucia, told tightly across the keep floors.
Church Road, Portchester, Fareham PO16 9QW
With one day in Portsmouth, the Historic Dockyard wins without argument. Portchester is a half-day for those staying two days or more, or with a particular interest in Roman history; the walls are free, so if you are driving on to Southampton or Winchester you can stop for an hour without paying for the keep.
The grounds, absolutely: flat grass, benches, room for a ball or a kite, and the pushchair rolls in over the bridge. The keep, no: a steep stone spiral stair with no lift, and pushchairs stay in the inner bailey. There are no toilets inside the castle, only at the car park 200 metres away.