The Arch of Hadrian is a monumental gateway resembling – in some respects – a Roman triumphal arch. It spanned an ancient road from the center of Athens, to the complex of structures on the eastern side of the city that included the Temple of Olympian Zeus. It has been proposed that the arch was built to celebrate the adventus (arrival) of the Roman Emperor Hadrian and to honor him for his many benefactions to the city, on the occasion of the dedication of the nearby temple complex in 131 or 132 AD.
It is not certain who commissioned the arch, although it is probable that the citizens of Athens or another Greek group were responsible for its construction and design. There were two inscriptions on the arch, facing in opposite directions, naming both Theseus and Hadrian as founders of Athens. While it is clear that the inscriptions honor Hadrian, it is uncertain whether they refer to the city as a whole or to the city in two parts: one old and one new. The early idea, however, that the arch marked the line of the ancient city wall, and thus the division between the old and the new regions of the city, has been shown to be false by further excavation.
Ängsö Castle was first named as "Engsev" in a royal charter by king Canute I of Sweden (r. 1167-1196), in which he stated that he had inherited the property after his father Eric IX of Sweden. Until 1272, it was owned by the Riseberga Abbey, and then taken over by Gregers Birgersson.
From 1475 until 1710, it was owned by the Sparre family. The current castle was built as a fortress by riksråd Bengt Fadersson Sparre in the 1480s. In 1522, Ängsö Castle was taken after a siege by king Gustav Vasa, since its owner, Fadersson's son Knut Bengtsson, sided with Christian II of Denmark. However, in 1538 it was given by the king to Bengtsson's daughter Hillevi Knutsdotter, who was married to Arvid Trolle.
In 1710, the castle was taken over by Carl Piper and Christina Piper. Ängsö Castle was owned by the Piper family from 1710 until 1971, and is now owned by the Westmanna foundation. The castle building itself was made into a museum in 1959 and was made a listed building in 1965. It is currently opened to visitors during the summers.
The castle is a cubical building in four stores made by stone and bricks. The lower parts is preserved from the middle ages. It was redecorated and expanded in the 1630s. The 4th storey as well as the roof is from the expansion of Carl Hårleman from 1740-41. It gained its current appearance in the 1740s.