When can I play a Knight Card.
For example, when it's my turn, and I already have a knight card I have not played, can I use it if I do not have the robber on one of my hexes?
Or, if it's another players turn, and they place a robber one of my hexes, can I immediately counter them with one of my Knight cards, or do I have to wait until it's my turn to use it to remove the robber?