Any location designated as a stronghold should be difficult to penetrate, stealth being the best bet in normal circumstances.
Several key points to remember:
1) This place has access to a very crafty wizard in the form of one Manshoon. While his physical presence is not necessary, there are other ways to make it felt. His knowledge of magic and vast resources would virtually guarantee that the fortress is hardened against the vast majority of extraplanar travel. So no Teleport, Dimension Door, Etc. I noted many good ideas in previous posts concerning various magical defenses. The Zones of Truth were excellent, as were most other suggestions.
2) Your "Invasion from below" idea, where the players sneak in from the Underdark, is novel, and extremely dangerous. Here is one way to put do it: Allow rumors of some sort of political upheavel, prisoner transfer, or whatever, to reach the ears of the PCs. Set a time limit. Push the PCs into the journey with little or no time to prepare. Make the trip through the underdark hazardous but not perilous. See to it that the characters are at about 50% resources when they arrive. Make it as nerve-racking as possible. Stress how tired and worn they are, while in the same breath reminding them of the limited time. This prevents you from having to craft encounters for a fully stocked party. Make each encounter a breath-holding, cannot bear to watch type occurence. See to it that every roll makes the table of players pray to whatever gods are listening.
Once they get inside, allow them a brief rest in a secluded spot. They retrieve the prisoners, but then something goes wrong. They are spotted, someone notices the missing slaves, etc. The players are now fully loaded with spells and hit points, but the way out should be 5 times harder than the way in. The enemy knows they are there. Throw everything and the kitchen sink at them. Allow them to nuke a hundred scrubs, because that is how the Zhents would do it. Each spell wasted on fodder is one less to cast against the big guys. Place multiple beholders in their path out. The ideas and possibilities are nearly endless, and the DM in me quivers at the thought of such a high-strung, yet ultimately cool adventure. Remember to make the former slaves real hindrance, yet not a game-killer. The Exalted PC in particular should be especially wary of any potential harm coming to the prisoners. An Iron Golem or 4 should wear away crucial warrior hit points, while the spell casters in the party are forced to utilize their magic on small enemies. As a climax, you could have Manshoon himself appear, perhaps getting one spell off before the party reaches the underdark and the then-possible teleport to safety. Manshoon is a patient and cunning mage. His revenge would be slow and inexorable, allowing for a whole new campaign arc should you change your mind and decide to run higher level games.
