We find Sansa wandering around the courtyards of the Eyrie when she sees a sight she never imagined she'd see again: snow. She takes this opportunity to spend time in her natural element and build a snow castle. Not just any snow castle though - she builds Winterfell, or as close a version as she can from her memory. However, Robin comes in and spoils it when he knocks the castle over. Sansa gets upset, which causes Robin to throw a tantrum, to which she answers with a needed slap to his face. This is a nice little scene that reinforces Sansa's connection to her family and longing for her home. Sansa hasn't seen Winterfell since the very first episode, so it really has been a long time. This scene also shows that despite all the terrible events she has been a part of, she still has the heart of that innocent and naive girl from Winterfell, for better or worse. No matter how much has transpired, Sansa will always be a Stark of Winterfell. Given all this, it is completely understandable why she gets so upset at Robin for ruining it to the point of slapping him. It may seem small in comparison to all the misery that has surrounded her for so long, but that is what makes it more painful for her. Even a tiny replication of her home isn't safe from the destructive forces of outsiders. This is the tragedy of the Starks.
