References to Zagreb can be found as far back as the Stone Age, but it gained shape as a city only in the Middle Ages. Established on two hills set against the backdrop of the Medvednica mountains, the city on two hills continues to be Zagreb’s identity. Though the city on the two hills is now called the Old Town, it is Zagreb’s heritage focal point. The Old Town is filled with a number of churches and other heritage structures, but Zagreb Cathedral with its twin towers towering into the sky dominates the city’s skyline. Close by is St Mark’s Church whose distinctive feature is the sloped roof with colourful mosaics. The mosaics are intricately laid to depict the city’s ancient coat of arms. The church is also popular since it forms the backdrop for the weekend spectacle of the changing of the guard. Other popular places to visit on the twin hills include St Catherine’s Church, Museum of Zagreb and the medieval Lotrscak Tower from where cannon is fired at noon every day.