The Aqueduct of Segovia, Spain

This is the first and most imposing old structure that I have seen outside of North and South America. Here is a short explanation of the date and method of construction from Wikipedia: Aqueduct of Segovia.

As the aqueduct lacks a legible inscription (one was apparently located in the structure's attic, or top portion), the date of construction cannot be definitively determined. The general date of the Aqueduct's construction was long a mystery, although it was thought to have been during the 1st century AD, during the reigns of the Emperors Domitian, Nerva, and Trajan. At the end of the 20th century, Géza Alföldy deciphered the text on the dedication plaque by studying the anchors that held the now missing bronze letters in place. He determined that Emperor Domitian (AD 81-96) ordered its construction and the year 98 A.D. was proposed as the most likely date of completion. However, in 2016 evidence was published which points out to a slightly later date, by 112 AD. 

The first section of the aqueduct contains 36 semi-circular arches, rebuilt in the 15th century to restore a portion destroyed by the Moors in 1072. The line of arches is organized in two levels, decorated simply, in which simple moulds hold the frame and provide support to the structure. On the upper level, the arches are 5.1 meters (16.1 ft) wide. Built in two levels, the top pillars are both shorter and narrower than those on the lower level. The top of the structure contains the channel through which water travels, through a U-shaped hollow measuring 0.55 tall by 0.46 meter diameter. The top of each pillar has a cross-section measuring 1.8 by 2.5 meters (5.9 by 8.2 feet), while the base cross-section measures 2.4 by 3 meters (7.9 by 9.8 feet). 

The aqueduct is built of unmortared, brick-like granite blocks. During the Roman era, each of the three tallest arches displayed a sign in bronze letters, indicating the name of its builder along with the date of construction. Today, two niches are still visible, one on each side of the aqueduct. One of them is known to have held the image of Hercules, who, according to legend, was founder of the city. The other niche now contains the images of the Virgen de la Fuencisla (the patroness of Segovia) and Saint Stephen.

We visited the Aqueduct on the day of the Feast of St. John the Baptist, a national holiday.