Best Siege Team Leaders?

So, my PC's are coming up to a fortress in an upcoming session, with the Lord being filthy rich and powerful (comparatively to a regular Lord). In preparing his fortress in one of the most dangerous parts of the land, the Lord has provided specific training for his well paid guardsmen, and spends a large amount of gold paying them.

As such, I want to theory craft -The best- siege team leaders for this fortress. As an aside, the normal threat to the keep is invasion, not a group of PC's. There's trebutchets, catapults, ballista, and cannons.

I would like to have siege crew leaders be 5th level. Any class (or combination there of) is fair game, along with retraining. For quick reference, the calculations for attack are:

Direct Fire: normal attack roll (BaB + dexterity + misc. modifiers)
Indirect Fire: Bab + (intelligence mod or knowledge: engineering)

Anyone have any thoughts? Siege mage looks good, but the lack of BaB is a big turn off.

I think a 5th-level 'Negotiator' Bard could probably talk his way into the fortress and convince the guards to open the front door?

I'm more of asking what the classes for the siege engineers controlling the ballista and cannons would be like.

At this point I don't care about how the PC's infiltrate the keep, but rather once combat breaks out how the fortress handles an impending invasion as well as dealing with the party.

The Siege Gunner archetype looks awesome for the crew leaders. Full BAB, and free Siege Engineer feat at 4 so they don't actually need to spend a feat on proficiency.

Trebutchets and catapaults benefit from skill modifiers, so Skill Focus is awesome (and the Siege Engineer's lvl 2 feature). I don't see any other feats that add to knowledge skills. Mossy disk ioun stones would help if you can throw some money at it.

For the ballistae and cannons, what would help besides masterwork siege engines and weapon focus? (Masterwork transformation for the win!) You could have some crew members be flagbearers.

Things that can help all of the defenders

Prayer, bless, and bard song.

True strike is great with a siege weapon.

In my opinion, siege engines are more for offense than defense. With their cost, their reload times, and crew requirements, one is much better off with just a bunch of lvl 1 warriors with bows or crossbows. On a castle, their main use, I'd say, is to destroy any incoming siege towers (bigger damage per shot to bypass hardness). If you have them, you may as well aim them at people, but not the most effecitve use of currency to defend one's town.

There's a gunslinger archetype that focus on Siege Weapons, i would not know if its what you are looking for but here it is:

http://www.d20pfsrd.com/classes/base-classes/gunslinger/archetypes/paizo--- gunslinger-archetypes/siege-gunner-gunslinger-archetype