The Cerro — a 139-metre hill on the western side of the bay — offers the best views of Montevideo and the Río de la Plata from its summit. The fortress at the top, built in 1811 and completed under various armies over the following decades, is now a military museum (Museo Militar) with exhibits on Uruguayan military history and the history of the fortress itself.
Getting to the Cerro requires a bus from the old city or a taxi — it's about 5 km from the centre across the bridge over the port entrance. The walk up from the bus stop is manageable. On a clear day the views take in the bay, the port, the city, and the wide flat water of the Río de la Plata stretching to the horizon. The combination of the views and the historical atmosphere of the fort makes it one of Montevideo's better half-day excursions.