If you missed a free window, Anno 1800 is a staple of seasonal sales. It regularly drops to for the base game. For that price, you get one of the most beautiful city-builders ever made, featuring:
Unlike older RTS games, you can zoom in to see individual citizens attending a world fair or walking through your botanical gardens.
Searching for a free "full version" download often lands you on sites offering cracked copies of the game. However, Anno 1800 is a massive, complex simulation with heavy and constant live updates. Downloading unauthorized versions comes with significant trade-offs: