Sure Mogror has found nothing. We haven't started yet, but till next inspection (if Mogror will bother himself with such boring and miserable tasks) we have to make hidden entrance.
So I suggest such design.
At the main tavern hall we dig stairs underground. To hide it, build 3x3 bridge above it. For those who ask - it's dance floor/boxing and wrestling ring/artist scene and so on. Bridge is linked not to lever, but to presure plate designed exaclty to horse weight. So, it works as follows - horse is pitted over plate and opens stairs. When door is opened, horse leaves and bridge closes.
For security reasons, we must install menacing spikes next to the door. Like this:
#####
#^D|D.
#####
^ - plate, D - doors, | - spikes
Doors and spikes are linked to levers. Lever that opens doors should be placed at tavern hall, lever that retracts spikes - at Peregarrett quarters. Also link something innocent to those levers, like hatches that cover holes at the roof. It's important to keep it secret.
For example:
Mogror: - You installed there a lever? what for?
Me: - It opens and closes hatches at the roof. We have to get used with sunlight, but we need to do it step-by-step. The lever regulates how much sun light you can get.
Mogror: - uh, ok then.
If he'll be suspicious and notice it's linked somewhere else - he'll have to follow that wire to the doors.
Mogror: - Open it! Pull that lever!
Me: pulling it, doors opened.
Mogror: Aha! a plate! Let's inspect that does it do...
He proceedes to it, ant then I pull it again. Now he's locked inside. The only way out for him is break the linkage and oped doors manually. But just when he proceedes to deconstruct it, I'm walking to my room and begin pulling my lever maniacally, hitting him with spikes, and slamming the hatch above me - in case Mogror begins his inspection from my lever, it diverts him from real purpose of it.