[The ice castle makes him pause for perhaps a second or two, at most -- but the video of her looking distressed is enough to push away the astonishment that she is actually using her magic to make something like this (passingly, Bucky wonders if it's strange to feel proud of her, thinking back to the conversation where he told her she could control it, if she just tried, and she can, now) and instead make quick work of climbing the steps to the castle.
He doesn't bother knocking, pushing open the door and almost calling out to her, but it's the sound of ice crystals forming that catches his attention, and he follows the sound, knowing it'll lead him to her.]
... Elsa. [If it's possible to look worried and happy at once, Bucky is mastering it right now.]