Alexander Dyce, A General Glossary to Shakespeare's Works

Castle “in Saint Alban's—The,” 2 HENRY VI., v. 2. 68. “The death of Somerset here accomplishes that equivocal prediction given by [the Spirit raised by] Jourdain the witch concerning this duke, which we meet with in i. 4. 35-37 of this play,— ‘Let him shun castles;
Safer shall he be upon the sandy plains
Than where castles mounted stand,’— that is, the representation of a castle mounted for a sign” (THEOBALD) .