The first Vasknarva order castle (Neuschloss) was founded in 1349 on the northeastern border of Old Livonia. 1427–1442 a new castle (Vastne-Narva) was built, which became the centre of the vogtei of the Livonian Order. The castle was wracked in the Livonian War. Until the Great Northern War it was a fort of great importance, commanding the mouth of the Narva River. It has been known in Russian chronicles either as Syrensk or Syrenets. According the folklore St. Olga of Pskov narrowly escaped drowning when crossing the Narva rapids at Syrenets. Nowadays only parts of 3 meters thin walls have survived, mainly on the northern side.
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.