I use Oxygen for all my XML editing and if you have a DTD or schema it has some pretty neat autocompletion and error highlighting features, with that in mind I’ve created a basic DTD for the level definition file in PBE, you can grab it from here. Note that it’s not ideal, as you get spurious warnings for the fields that you specify inside your component definitions, but there’s not much that can be done about that.

Update: added an XSD version of the file, this handles unparsed content gracefully, useful for components.