I suspect the FE underground will be the castle's secret escape tunnel, as most FE castles tend to have them.
They featured the underground area in the FE anime as well.
As for the theory, bravo on the numbered bit, but forget the naming bit. It's irrelevant.
The point is, if you type, "http://www.smashbros.com/en_us/images/icon/world_b_01.gif", you get the smash icon, but if you change it to "/world_b_02.gif", you'll get the next icon in the list, but there ARE icons missing, mainly 07, 13, 14 and everything beyond 18.
The fact that there are numbers missing and yet the sequence continues raises a perfectly valid point because that means these numbers are, without a doubt, placeholders for a symbol which has wisely not yet been added to the site (since people would then find it).
There is no naming order, but the fact that these are placeholders for later reveals cannot be denied...