There is only one (nonmagical) way in/out of the stronghold, and the rooms the party rests in usually block the "exit".
The first night, invisible (and hidden) enemies kept trying to pass by, annoying the PC on guard until they stopped guarding the hallway and kept watch in the room. (Yes, I'm serious!)
Another night the group slept in a room at the end of little-used hallway. Again there was no guard outside the door, so they awoke to find that the hallway had been walled up (with a little bit of magical help). They got out of that fairly easily, but still haven't learned a lesson.
Other things I've done is have traps reset, put summoned creatures outside the door, etc.
A couple more items of note:
* Divination, illusion, and "deceptive" spells do not function well. Detect Magic, Invisibility, etc have roughly a 75% chance to fail (without a proper token)
* The characters are 7th-8th level. There's a Wizard (evoker), Druid, Barbarian, Rogue, and Sorcerer. Healing magic is fairly limited.
* The party has established that it excels at wiping out "grunts", so using sheer weight of numbers to whittle down the PCs would break the attackers' morale quickly. (The standard grunts are CR 2 (Barbarian 1/Warrior 2), so they don't fall in one hit).
* The grunts (and leaders) learn from the group's tactics, and revise their strategies. For example, to prevent the PCs from charging, the first rank of grunts usually carries longspears (for the Attack of Opportunity), and archers fire to disrupt spellcasting. When the group started using Reverse Arrows on themselves, the archers eventually either scrounged a few +1 arrows, or shot at the PC's mounts.
I'm running out of ideas of "unkind" things to do to them while they sleep in the mound. Keeping them up all night (noise, nightmares, etc) so the spellcasters can't rest is something I'm saving for later. Wiping out the party while they sleep is not an option.