Basel, also known as Basle, is a city in northwestern Switzerland on the River Rhine. Basel is Switzerland's third-most-populous city, with 177…Basel, also known as Basle, is a city in northwestern Switzerland on the River Rhine. Basel is Switzerland's third-most-populous city, with 177,595 inhabitants within the city municipality limits. The official language of Basel is Swiss Standard German and the main spoken language is the local Basel German dialect.