The Great Mosque of Kairouan was built in 670 AD and is one of the oldest places of worship in the Islamic world. It is now a UNESCO World Heritage Site and is a major pilgrimage destination. The mosque is 100% free to enter, but you may need to tip the guy looking after your shoes. If you are not appropriately dressed, the mosque will provide coverings for free. The mosque covers over 9,000 square meters and features a stunning courtyard, a square minaret, and other intricate architectural details worth taking a moment to appreciate.