St. Mark's Clock Tower in Venice is an early Renaissance building on the north side of the Piazza San Marco, at the entrance to the Merceria. It comprises a tower, which contains the clock, and lower buildings on each side. Both the tower and the clock date from the last decade of the 15th century, though the mechanism of the clock has subsequently been much altered.
It was placed where the clock would be visible from the waters of the lagoon and give notice to everyone of the wealth and glory of Venice. The lower two floors of the tower make a monumental archway into the main street of the city, the Merceria, which linked the political and religious centre (the Piazza) with the commercial and financial centre (the Rialto).
The first clock housed in the tower was built and installed by Gian Paolo and Gian Carlo Rainieri, father and son, between 1496 and 1499, and was one of a number of large public astronomical clocks erected throughout Europe during the 14th and 15th centuries. The clock has had an eventful horological history, and been the subject of many restorations, some controversial.
Stairways (steep and narrow) inside the building give access to the terrace on the roof, passing the clock mechanism on the way. Tours (in Italian, English and French) must be booked in advance.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.