A dam was built in 13th century in the river Amstel (that's why the city is called Amsterdam). The old harbor of the city, "Damrak", is now one of the busiest streets in the city. The city was for the first time mentioned in 1275 by count Floris V, count of Holland and Zeeland who gave the people of Amsterdam some more freedom. The city got its city rights probably in 1306; at least soon after the year 1300. The city became a major trading port pretty soon, with a successful trading route to the Baltic Sea. The city grew fast in the 15th century. Because the city basically lies in swamp, the builders of the city had to dig canals to use the digging-ground for getting the living-ground higher. Homes were built on wooden poles, and the canals served as a pretty effective primitive sewer.