        In the evening, she would spend a couple of hours with Princess Celestia, practicing Starswirl’s gate spell, as well as weaving other spells with it, to be sure of its stability.  Celestia occasionally threw offensive spells at the shield, to see if they would affect the underlying matrix, and when that produced no unexpected results, began throwing physical objects at it.  With the help of the Unicorn Guards, they also tested the fail-safe mechanism of the spell, collapsing it when somepony was at the destination, as well as when somepony was partway through.  In all cases, the traveler was returned unharmed to the starting point.  Twilight felt like a foal again, practicing spells alongside Princess Celestia.