Why is 'Ellos se fueron a un castillo' incorrect?
'ir' means to go and 'irse' means to go away, or leave, if that makes a difference "Mis padres se fueron al campo" = my parents went away to the country.
Could you also leave out 'ellos' so that it's: Fueron a un castillo?
Yes!
isn't there a corrent past form of "va"?