Invisibility

Invisibility’s multitude of useful functions marks it as one of the most valuable spells for a stronghold builder. You can use it to make protected windows by casting it on a spot in the outer wall of your stronghold. The affected part of the wall remains as strong as the original material, but becomes entirely transparent. Cast on your roof, it provides a skylight that never leaks (unless your roof does).

Cast invisibility on a case protecting a valuable item on display, such as in a museum. Instead of a fragile glass case, you can now have a durable iron, mithral, or adamantine case protecting the item.

Use invisibility to create peepholes in doors. Cast on a dividing wall, it creates a transparent barrier between you and any visitors to your main hall (and it’s cheaper than wall of force). If the wall has an arrow slit, it grants an archer far more cover than apparent while still allowing a wide field of vision (though the archer’s field of fire remains restricted by the arrow slit).

Casting: 60 gp (Clr3 or Wiz3) or 80 gp (Brd4 or Sor4)

Permanent: 5,500 gp (10th)