Casco Viejo is the old town of Bilbao, a city in northern Spain. It is located on the east bank of the Nervion River and is known for its charming narrow streets, historic buildings, and lively atmosphere. The area is one of the most popular tourist destinations in the city, and it's easy to see why.

The heart of Casco Viejo is the Plaza Nueva, a beautiful square surrounded by arcades that are home to numerous cafes, bars, and restaurants. It's a great place to sit and people-watch while enjoying a coffee or a cold drink. From the square, visitors can explore the surrounding streets, which are lined with shops selling traditional Basque crafts, clothing, and food.

One of the most iconic buildings in Casco Viejo is the Santiago Cathedral, a Gothic-style church that dates back to the 14th century. The cathedral is located on the Plaza del Santiago, a small square that is also home to a weekly flea market.

Other notable buildings in Casco Viejo include the Arriaga Theatre, a neoclassical building that hosts concerts, operas, and plays, and the Mercado de la Ribera, a large covered market where visitors can find fresh produce, meats, and seafood.

Casco Viejo is also home to many pintxo bars, which are a type of Basque tapa that are typically served on small slices of bread. It's a great place to sample some of the region's most delicious and iconic dishes, including bacalao (salt cod), chipirones en su tinta (squid in ink sauce), and txipirones (grilled squid).