This spell throws up an invisible force shield that does not allow any creature to enter the room. However, players in the room may freely leave. Once they leave, they can't get back in until the spell duration is over. The duration of the spell is based on your level. A level 20 cast of this spell can last up to 5 mins. The spell also helps the regeneration of all who are in the room, if it isn't a castle room. If there is an imprisonment spell working in the room, a shelter will destroy its effects.