You can use the advancing darkness to duplicate unique buildings!
You can get multiple Mainports (see below). You can also get multiple Magical Village Level 2s, Towers of Water, and Magical Cities (the one next to the Tower Of Light). You can also get both a Mushroom City AND a Mushroom Port City, and both a Tree Roots City AND a Tree Roots Port City.
Darkness? More like Emergent Gameplay lol