An emergency escape spell that allows the Summoner to immediately swap positions with a Summon Servant. Because Castling only requires a blink of the eye to activate, it is possible to use it to evade attacks right before they reach the Summoner.
However, Summoners who use this skill in an absolute crisis must be prepared to sacrifice the Servant. It is useful in a combination with Soul Possession, usually used to get past terrains that are difficult to traverse as a human.
Go East arc
Route 43 arc
Roe2 told the kids she used Castling to switch places with her brother to get to Oou from "far away" (i.e. the moon).