I think Wizard Lock on the base would be awesome. And in case of silence, we might set multiple entry methods, like a pass-phrase and a hand-sign. Man, I remember I was playing a 4E game and we were talking about putting an Arcane Lock on our base and it was possible to specify certain people to bipass it like the original caster can, that would be best, but alas. Perhaps an item in hand allowing us to bipass it? Just a certain way of opening the door be the hand-sign? Like if you open then door while crossing your pinky and ring finger or something. Whatever we decide I suggest we have Curtis cast it, since casters 3 levels higher than the original caster can get in without even a spell, and as far as I know he is the highest level caster we have. For some reason, talk of a secret fortress made me immagine some like hidden underground place, like the ground looks totally normal until you speak the pass-phrase, then a square of earth raises slightly and we lift it and walk down the stairs, pulling it shut behind us.