You must use the slider to select troops to go and help defend another castle. The traveltime indicated will always be based on the speed of the slowest unit.The troops will be deployed when you press the “Support” button.
External troops will be listed separately in the troop overview of a castle and, once they have arrived, they can be called back at any time. It may be very useful to send troops to other castles, for example after capturing a castle or in the event of an imminent attack.