Another landmark is the old centre and the canal structure in the inner city. The Oudegracht is a curved canal, partly following the ancient main branch of the Rhine. It is lined with the unique wharf-basement structures that create a two-level street along the canals. The inner city has largely retained its Medieval structure, and the moat ringing the old town is largely intact. Because of the role of Utrecht as a fortified city, construction outside the medieval centre and its city walls was restricted until the 19th century. Surrounding the medieval core there is a ring of late 19th- and early 20th-century neighbourhoods, with newer neighbourhoods positioned farther out. The eastern part of Utrecht remains fairly open. The Dutch Water Line, moved east of the city in the early 19th century required open lines of fire, thus prohibiting all permanent constructions until the middle of the 20th century on the east side of the city.