You can cast the ritual just fine already.  It goes off without incident outside the lab and cover the entire thing if you make the die rolls for it.  Just get on with it already. (though the ritual doesn't hit the parts of the lab in Khyber, if there are any, so if you're really paranoid about that, you could drop it once you find the dug too deep place)  The engineers checked over the area for new breaches and found suspicious shifting in foundations which might indicate a new tunnel being dug out of the facility.  If the tunnel reaches the surface, it could turn the self destruct system into a doomsday device, flooding the town with magma.<br/><br/>As long as the die rolls for the ritual come off, you'll get it off without incident, since the undead are contained and don't have any means of watching what you're doing, though completing the ritual will freak them out pretty bad.  Entering from the side entrances won't make too much of a difference, except in possibly letting you bypass any undead sent to greet you.  The army cannot muster a hide from undead spell for everybody in the attack force though, so if you want to go in hidden, you're going to be limited to about a 20 person strikeforce.<br/><br/>Does that make you happy?  I'd kind of like to gloss over things and get to the action.  Is that so wrong?  If your characters want to retroactively do intelligent things, that's fine, but you don't exactly need to be sun tzu to fight zombies.  In fact, it's counterproductive, since most tactical efforts don't really help against large masses of mindless undead, since they don't tend to break and run when you flank or envelop them.  Though to be fair, undead are extremely prone to walk mindlessly into deathtraps.