Can anyone explain this expansion please? When you purchase a Sultan card, what exactly are you selecting? Is it a colour or a type of building? If a colour, can you automatically pick up all buildings of this colour, when they become available? If a type, what exactly does this mean? A building with a wall on the bottom edge for example? Or must this be specifically a building with a bottom edge that is in the colour that you have selected? I am really confused here!
I should point out up front that this expansion is one of the few that I have not yet played with, so someone should feel free to correct me if I screw anything up here. Nevertheless, here is how I understand it to work.
When you purchase a Sultan card, that card will allow you to buy one building of a certain type free of charge (note all buildings of a certain type have the same color - for instance, all of the Gardens are green). So, let's say for instance that a Sultan card that you have purchased has a black cube on the Seraglio's (the red buildings). When the builder's yard is being replenished at the beginning of a new turn, if a Seraglio that you want appears, you can use the Sultan card to buy that building free of charge. You only get to use the card once (the Sultan card that you used to take the building gets discarded), and you only get to buy one building. The wall structure does not matter - you could take any red building regardless of the wall structure.
The purpose of the Overview card is simply to show the distribution of the types of buildings. It could also help you know whether or not a certain Sultan card would be of any use to you. For instance, when you re-roll the building die after acquiring the Sultan card, you probably don't want to place the marker on the Gardens if most of them are gone.