Fort Hoofddijk was built in 1879 as one of the fortifications around Utrecht that formed part of the Nieuwe Hollandse Waterlinie, or 'New Dutch waterline'. This defense line made use of inundating parts of Holland to stop the advance of the enemy, but because Utrecht was on relatively high ground, it needed an additional ring of fortifications.
Today the Botanical Gardens of Utrecht University are located at the fortress site. It’s a beautiful place for a long walk along rare species of plants and flowers.References:
The city walls of Avila were built in the 11th century to protect the citizens from the Moors. They have been well maintained throughout the centuries and are now a major tourist attraction as well as a UNESCO World Heritage Site. Visitors can walk around about half of the length of the walls.
The layout of the city is an even quadrilateral with a perimeter of 2,516 m. Its walls, which consist in part of stones already used in earlier constructions, have an average thickness of 3 m. Access to the city is afforded by nine gates of different periods; twin 20 m high towers, linked by a semi-circular arch, flank the oldest ones, Puerta de San Vicente and Puerta del Alcázar.