Colonia del Sacramento is in southwestern Uruguay, across from Buenos Aires on the northern shore of the Rio de la Plata. Founded in 1680 as a Portuguese colony, Colonia's historic district is now a UNESCO World Heritage site, full of cobblestone streets lined with historic buildings.
Its old buildings and cobblestone streets preserved from the colonial days make it a popular destination for people from Buenos Aires on the opposite side of Rio de la Plata. It's especially popular as a weekend getaway for couples or as an attractive stop on a trip between Buenos Aires and Montevideo.
The main attraction in Colonia is its historic center, the lower part of the town which is a UNESCO World Heritage Site. It consists of lots of old buildings, many from the Portuguese colonial age. It's a lovely area to wander around. The old fort has played an important role in the wars against Argentina and Brazil, and the old city wall is worth seeing.