As the Leader or an Officer, you can Target a castle to put a large targeting icon above it, showing your teammates key locations. When you create a Target, you're given the option for a red or a green (presumably to show hostility or friendliness) for the target, as well as an optional note. A message will be sent to team chat with the note. Your team can set a limit of 10 total targets.
Anyone on the team can bookmark locations for quick access from the team. An unlimited number of bookmarks can be set.
Once bookmarked, all bookmarked locations can be found in the Castle List button under the [...] button on the right.
Tapping on the small arrow icon takes you to the location.
Bookmarks can also be removed by simply going into the location's 'Details' page and pressing the "Remove Bookmark" button at the top of the screen. Do this once in a while to keep the list from getting messy!