PCs vs Siege Engines

[Dalmosh]

Is there any way to reasonably ballpark Challenge Rating for siege engines when used directly against PCs?
Or do you just treat an NPC operating a cannon as having that as part of their equipment and go from there?

Lets say the party is facing up against a statted out war machine from DMG or Heroes of Battle, crewed by level 1 warriors... how do you represent that as an encounter balanced to party level?

Would you treat that kind of encounter like a trap, and approximate it compared to trap damage?

[DeTess]

I'd treat it as a trap of sorts. As long as the siege engine can hit the pc's, the pc's should be out of range to retaliate, and once the pc's get close enough to retaliate, they'll have underrun most of the big rockthrowers.

If you're using a ballista or other direct fire machine I'd treat it as a normal combat encounter and count the machine as equipment.

[Melcar]

Is there any way to reasonably ballpark Challenge Rating for siege engines when used directly against PCs?
Or do you just treat an NPC operating a cannon as having that as part of their equipment and go from there?

Lets say the party is facing up against a statted out war machine from DMG or Heroes of Battle, crewed by level 1 warriors... how do you represent that as an encounter balanced to party level?

Would you treat that kind of encounter like a trap, and approximate it compared to trap damage?

Considering attack is based off of the chief engineers skills and level, I would base it off the crew's level.