Question about “Cities & Knights” Expansion Progress Cards: Diplomat - When is a road open?
A road segment is open when it is located at the end of a continuous road. As soon as your continuous road connects two of your settlements with each other or one of your settlements with one of your knights, the continuous road is considered as closed. An individual road piece of a closed continuous road may not be moved. This rule also applies if an opponent builds a settlement or knight on an intersection of your closed continuous road. In this case, the continuous road is considered as interrupted, thus reducing its length counted towards the Longest Road; however, the two settlements or the settlement and the knight are still connected, which is why the continuous road remains closed.
In this picture, all open roads are marked with an asterisk: