Count of CarcassonneG 20

Start a single game with one opponent on easy. Avoid the expansion pack River. The King and Barons expansion pack can be bought on the marketplace. I couldn't get this one without the expansion packs, so I bought the King and Barons pack and I had the achievement the first time I played.
You'll get more points with the King and Barons pack turned on. Make sure you are King (make the biggest castle) en Baron (make the biggest road) because at the end you'll get extra points for every finished castle (if you are King) and extra points for every finished road (if you are Baron)

Edit: You have to "finish" the biggest castle and road to become King and Baron, so make sure you put the last piece. It is not necessary to have a follower on it when finishing the castle or road.