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.
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).
Steppe is a very open map without any mountain or lake tiles. There are many steppe tiles that don't provide any resources, and there is a significant lack of forests, with only two scoutable forest tiles.
Fjords is a very enclosed map with many mountains and water tiles. Resources are plentiful, but with six mountain and six water tiles, you'll have to be patient when traversing this terrain.