Servants. Innocent people who would give them away if they could, but whom the party would feel guilty killing (and killing them could lead to nasty repercussions as well).
If the king is evil, then traps (possibly in fake, otherwise unused rooms or halls) to kill intruders, if not evil, then capture traps (even if he's evil, he might want to question his enemies).
Of course, golems or other automatons could be part of the defenses; it is easy to give all authorized folks some insignia or password to get by them.
Live guards; they may be bribable, but are less likely to be fooled by imposters than the automatons would be.
The king's mistress, wife, personal servants, etc...
Greedy/clever nobles or other court members who find the party and wish to hire/bribe/blackmail them into doing some deed they didn't originally intend. Imagine sneaking in to the castle intending to spy on the king, and getting recruited to kill him or his chief advisor or high wizard or someone in the process.
That's all I can think of now.
Gilladian