Wikipedia:

The Dam derives its name from its original function: a dam on the Amstel River, hence also the name of the city.[1] Built in approximately 1270, the dam formed the first connection between the settlements on the sides of the river.

As the dam was gradually built up to it became wide enough for a town square, which remained the core of the town developing around it. Dam Square as it exists today grew out of what was originally two squares: the actual dam, called Middeldam; and Plaetse, an adjacent plaza to the west. A large fish market arose where ships moored at the dam to load and unload goods. The area became a centre not only of commercial activity but also of the government, as the site of Amsterdam's town hall.

As a market square, the Dam had a weigh house that can be seen in some old paintings. It was demolished in 1808 by order of Louis Bonaparte who, upon taking up residence in the newly converted Royal Palace, complained that his view was obstructed.

The Damrak, or the former mouth of the Amstel River, was partially filled in the 19th century; since then, the Dam square has been surrounded by land on all sides. The new land made room for the Beurs van Zocher, a stock exchange that was built in 1837. After the stock trade moved to the Beurs van Berlage in 1903, the Zocher building was demolished. In its place, De Bijenkorf department store has stood since 1914.

In 1856, a war memorial named De Eendracht (The Unity) was unveiled inside the square before King William III. A stone column with a female statue on top, the monument acquired the nickname "Naatje of the Dam". It was taken down in 1914.