Merkator is about the rise of Hamburg after the Thirty Years' War.
You visit cities to collect goods or fulfill orders. The collected goods are added to the cities when a player visits a neighboring city. Fulfilling an order provides you with another better, but more complex order additional to the fulfilled order which you keep and can fulfill again, although the number of orders you can own is limited. Each order itself is worth a certain amount of points at the end of the game. Also you can exchange these points for special cards which provide additional goods in certain cities or more victory points if you fulfill certain conditions at game end.
Depending on the city you want to enter you either receive a resource called "time" or you have to spend it. By paying a certain time-fee you are allowed to accompany another player on his trip to a town to fulfill your orders in this town (but not to collect goods). The game ends when a player receives the order with the highest value by fulfilling the order one level below.