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 Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.