To say that a summons from Zachariah was like being called to the principal's office would have been over simplistic; to Castiel he was not just a superior. He was a brother, first and foremost – his elder brother, to be specific. After that, a mentor, and after that a fellow warrior – a brother not just by blood (or whatever passed for 'blood'; angelic relations were complicated, comparable to and yet at the same time completely unlike human ones) but in arms, one of the few remaining soldiers in their once-proud garrison. They had stood together when God commanded that the half-blood children of the fallen Grigori be destroyed, and again when they besieged hell to rescue the elder Winchester, and a hundred other times besides, and Castiel had fully expected that the same would be true when they prevented Lucifer from rising.
Zachariah was all these things. He was also a douchebag. The word still sat uncomfortably on Castiel's tongue, but he could not deny that his brother's behaviour had been... confusing, to say the least.
The same, of course, could be said of his own.
The summons was not entirely unexpected, taking that into account; if anything he was surprised not to have had one sooner. Zachariah had been busy, he supposed - doing his job, a sharp voice reminded him, a shrill, guilty stab. Resisting would have been both futile and idiotic, though to say the trenchcoated angel came willingly might have been a stretch – if Zachariah was the elder brother, then Cas played the wounded younger one to a tee, trailing his feet and keeping his head bowed as he was escorted to the Greenroom. The significance of that venue did not escape him. This was Zachariah's bastion.
Castiel did not anticipate this being a pleasant exchange.
He did not keep his head down for long once the familiar walls surrounded him, nor pause to admire the paintings; his gaze met Zach's evenly, that childish reticence now an equally chidlish determination not to seem guilty. “Brother. I was sent for.”