The taking of Castle is really poorly designed. Preston starts going on and on about it but it's actually quite hard and he leaves Sanctuary when you give into his nagging and you can't hand in settler quests until you complete.
Also Preston was killed during the Queen battle and as a result respawned in Sanctuary and then had to run all the way back to Castle, which meant the quest was broken but was paused in a non-sensical way for about ten minutes.