Vagrant Story has a great many locked doors. Some are sealed with magical Sigils, others with hidden keys. Both function the same way - you need the key or Sigil in order to get through a door. Here's a quick list of where all of them are, so if you don't have one of them, this will give you at least an idea of what you have to do. The Sigils are mostly dropped by major enemies, so chances are you'll be getting all of them anyway, but some are in chests that it's possible to miss if you don't keep an eye out for them. Anyway, here it is. Notice that the Sigils are all curiously named after plants.