Possible Siege defending exploit? Or just a sneaky trick?

[PBI]

Am i posistioning them so the trajectory is impossible

Probably; high trajectory artillery like trebuchets and mortars can normally fire out at the attackers just fine and are the only arty worth their salt on the defensive.

Is there an optimal posistion for Handgun type infantry on rampart walls?

In a word, no. The optimal position for gunners is anywhere but the walls, they just don't fire properly due to being obstructed by each other and the crenelations. Unless you mean the actual hangunners, in which case they are OK as wall guards due to their decent melee stats. Won't kill anyone with their guns though.

ATPG, you could try doing a defensive campaign, where you simply don't attack any other factions and do everything you can to make them hate you, without actually taking any cities, just trying to keep your original holdings. I tried it a while ago as the Byzantines; it actually got quite tricky once all my neighbours were at war with me, I basically had three fronts and only enough money for one, maybe two field armies. Lots of good skin-of-my-teeth type defensive siege victories against enormous odds. I guess the extreme turtling style required might be anathema to your blitzing sensibilities however.