Beaumaris Castle (June-Sept daily 9.30am-6pm; April, May & Oct daily 9.30am-5pm; Nov-March Mon-Sat 9.30am-4pm, Sun 11am-4pm; A?2.50; CADW) might never have been built had Madog ap Llywelyn not captured Caernarfon in 1294. When asked to build the new castle, James of St George abandoned the Caernarfon design in favour of a concentric plan, developing it into a highly evolved symmetrical octagon. Sited on flat land at the edge of town, the castle is denied the domineering majesty of Caernarfon or Harlech, its low outer walls appearing almost welcoming until you begin to appreciate the concentric layout of the defences protected by massive towers, a moat linked to the sea and the Arab-influenced staggered entries through the two gatehouses. Despite more than thirty years' work, the project was never quite finished, leaving most of the inner ward empty and the corbels and fireplaces built into the walls unused. You can explore the internal passages in the walls but the low-parapet wall-walk, from where you get the best idea of the castle's defensive capability, remains off limits. Impressive as they are, none of these defences was able to prevent siege by Owain Glyndwr, who held the castle for two years from 1403, although they did withhold a Parliamentarian siege during the Civil War.

Almost opposite the castle stands the Jacobean Beaumaris Courthouse (Easter & June-Sept daily 11am-4.30pm, May Sat & Sun same times; A?1.50; joint ticket with gaol A?3), built in 1614 and the oldest active court in Britain. It is now used only for the twice-monthly Magistrates Court, but until 1971, when they were moved to Caernarfon, the quarterly Assize Courts were held here. These were traditionally held in English, giving the jury little chance to follow the proceedings and Welsh-speaking defendants no defence against prosecutors renowned for slapping heavy penalties on minor offences. On session days you can watch the trials, but won't be able to take the recorded tour or inspect The Lawsuit , a plaque in the magistrates' room depicting two farmers pulling the horns and tail of a cow while a lawyer milks it.

Many citizens were transported from the courthouse to the colonies for their misdemeanours; others only made it a couple of blocks to Beaumaris Gaol , Steeple Lane (Easter & June-Sept daily 11am-5pm; A?2.50; joint ticket with courthouse A?3) which, when it opened in 1829, was considered a model prison, with running water and toilets in each cell, an infirmary and, eventually, heating. Women prisoners did the cooking and were allowed to rock their babies' cradles in the nursery above by means of a pulley system. Advanced perhaps, but nonetheless a gloomy place: witness the windowless punishment cell, the yard for stone-breaking and the treadmill water pump operated by the prisoners. The least fortunate inmates were publicly hanged, the fate of a certain Richard Rowlands, whose disembodied voice leads the recorded tour of the building and various displays on prison life.

After all this gloom, a good way to lift the spirits is aboard one of the pleasure cruises (tel 01248/810251 or 810379) on the Island Princess out to (but not landing on) Puffin Island. The booking kiosk is at the foot of the pier.