Abergavenny Castle was established by the Norman lord Hamelin de Balun c. 1087. It was built to overlook the River Usk and its valley, and so guard against incursions into the lowland areas south and east of the town by the Welsh from the hills to the north and west.
The castle was then the scene of an infamous massacre. Over Christmas 1175 De Braose called Seisyll and his son Geoffrey to his castle, together with other leaders from Gwent, supposedly as an act of reconciliation. De Braose then had the men killed in the castle's great hall, in retribution for the death of Henry Fitzmiles. His action, including taking the men's land, resulted in sanctions: William was 'retired' from public life and the castle passed to his son, William. In 1182 Hywel ap Iorwerth, lord of Caerleon, ordered the destruction of Dingestow Castle and had Abergavenny Castle set afire in retribution for the murder of Seisyll. The attacks were made by Seisyll's relatives. De Braose was not at the castle when it was burnt, but 'most of his men' were taken hostage.
The castle was almost entirely rebuilt of local Old Red Sandstone, starting about 1190, to make it easier to defend. Five towers were built along the curtain walls, and a keep was built. The English and Welsh fought for control of the Welsh Marches and, during this time, possession of the castle alternated between the Welsh and English. In 1215, the castle was visited by John, King of England.
In 1404, during the rebellion of Owain Glyndŵr, the town of Abergavenny was sacked and burned by Welsh forces. The fortified, or barbican, gatehouse, may date from either just before, or just after, that date.
The castle, now in ruins, had a stone keep, towers, and ditch as fortifications. It also housed the family and army of the lord and had cellars, kitchens, a great hall, gatehouse, and a chapel, although it is doubtful whether any of the families treated the castle as their main residence. A curtain wall surrounded the castle.
The high, formidable curtain wall, dating from the 12th century, is now the most impressive part of the ruin. A 19th century lodge was built on the top of the motte in the 19th century.
The castle's museum is located in the 1819 hunting lodge on top of the motte. Amongst the museum's exhibits are a saddler's workshop and a Victorian Welsh farmhouse kitchen.
Visby Cathedral (also known as St. Mary’s Church) is the only survived medieval church in Visby. It was originally built for German merchants and inaugurated in 1225. Around the year 1350 the church was enlarged and converted into a basilica. The two-storey magazine was also added then above the nave as a warehouse for merchants.
Following the Reformation, the church was transformed into a parish church for the town of Visby. All other churches were abandoned. Shortly after the Reformation, in 1572, Gotland was made into its own Diocese, and the church designated its cathedral.
There is not much left of the original interior. The font is made of local red marble in the 13th century. The pulpit was made in Lübeck in 1684. There are 400 graves under the church floor.