City gates are structures that mark the entrances and exits of a city. Historically, they served as important defensive features, protecting the city from invaders. Many city gates were built with thick walls and towers, often adorned with decorative elements. They also acted as checkpoints for trade and travel, controlling the flow of people and goods. In modern times, some city gates have been preserved as historical landmarks, showcasing architectural styles from different periods. Examples include the Brandenburg Gate in Berlin and the Porta Nigra in Trier. These gates often attract tourists and serve as symbols of the city's heritage.