I have to agree with DCCFan. The slavers face a tough choice; capture/kill the party or relocate their operation. The best possible outcome for them would be to capture the party and sell them to the orcs (earning them a profit and removing a pesky problem at the same time).
However, don't forget that Ikenvar's motivation is to rebuild
the Iron Manacle; if they take too many losses, their operation will suffer and they face the possibility of the orcs taking THEM as slaves, or simply killing them outright.
Thus I would play the slavers as sneaky and ruthless as possible with the following objectives:
1. Capture the party.
2. Kill the party if #1 is impossible.
3. Flee if #1 and 2 are impossible.
The 12 hour wait gives the rogue ample opportunity to sow traps in the tower that will weaken/separate the party. While the traps/smoke/whatever will serve to weaken them slightly, it will also irritate them. You want the party to hate
the slavers; it will make it all that much more satisfying when they finally beat them.
Worst case scenario; the party dies. The players learn a lesson and pick up new characters.
Second worst case; the party is captured and sold to the orcs. Then you have a choice; continue running the adventure with these characters as slaves attempting a Prison Break scenario OR they pick up new characters who will "unlock" the pre-gens once the slavers and orcs have been defeated.
Best case, they fight their way out of it and win. (The monk, of course, escapes!)