Does this mean I'm an enemy of the Brotherhood? Where did I go wrong? The most Brotherhood related thing I did during the quest was call a vertibird, but that was it. Or was it because I'd left Danse in the castle?
Completing this quest fails Tactical Thinking. As Nukapedia says:
Completing this quest will automatically fail Semper Invicta, Ad Victoriam, Liberty Reprimed, Blind Betrayal, and Tactical Thinking. However, completing Ad Victoriam while Defend the Castle is active will successfully complete Defend the Castle.