Question
How can I build a secret base without having to just kill all the builders afterwards to keep the secret?
Explanation
Another player and I want to build a secret base for an assassins guild we are forming in our E6 game, but neither of us are good at masonry or anything like that. I am not sure if something like an unseen servant could help us because if I am not mistaken, they do not make the check themselves, we have to make the check for them. If we use NPC labor they would know the secrets of our base and would probably need to die... which could get messy because they may expect an attack if we make them build too many secret passages and stuff.
Our Chars
Both are basically Stealthy Combat Characters: a Rogue and a Shadow Caster. Both are level 6 with around 3 post-6 feats or so. My Rogue is working on getting Shaper Psion class features but will not be able to get more than what a level 6 Psion could.
Restrictions
We are playing D&D 3.5e using the E6 variant, which basically caps out the characters at level 6 though they can still gain feats.