User blog comment:Martin2/Essay:Unanswered Questions of Redwall (Rebuttal)/@comment-1523141-20100304233143

I'm fairly certain that Salamandastron gets it from the cooled lava in their volcano, there are many forges that cool magma and trim the solid rock, chip away and then form iron, as the center of the earth is mainly molten iron. As for southsward, it's possible that they have mines and forges in the castle.