The spell was cast by a mage from Silverymoon called Lancier Arballister. Akordia and Jason arranged for him to perform the ritual, checking his credentials with the church of Mystra in an effort to ensure he was trustworthy. The gateway is guarded both ends and is large enough to allow a horse to pass through, although the tricky part would then be getting a horse out of the basement in Stanzfeld Mansion!