Highland is the starter map. It contains many resources, a few mountains and lakes, and two villages. This map can support both aggressive and defensive play styles, depending on the layout of the tiles (with obstacles in the middle or on the sides).
Constantinople is the second map in the base set. The middle tile is always the city of Constantinople, providing double gold resources. There are two tiles representing the Black Sea and the Sea of Marmara. Giving a lot of potential for naval battles.
Constantinople is the only map that doesn't have a 4x4 version.
Fjords is a very closed map with lots of obstacles, including mountains and numerous water squares. This makes it a great map for civilizations that are more naval-focused while also providing strong natural defenses for slower-moving civilizations.