New GM looking for advice on a campaign idea

[nikolamat]

My campaign (set in Golarion) idea is to have the PCs stop an invasion of orcs from destroying Korvosa. My idea is that Magnimar has paid off some mercs and orcs to destroy Korvosa. Also they have given them muskets, cannons and gunpowder (which are very recent discoveries in my campaign, which korvosa still doesn't have access to). They are all in smaller camps scattered across the bloodsworn Vale with one big camp also there. Most of the gunpowder muskets and musket schematics are gathered in a big camp guarded by the orcs ( the mercs and the orcs are in different camps to avoid conflict). The PCs objective is to blow up the gunpowder (effectively destroying most of the army) and scavenge some muskets and musket schematics. My question is: Is this scenario viable and how many orcs/mercs would it take to pull off a large scale invasion of Korvosa? Since this is my first campaign there are probably major problems so feel free to point them out.

[erikun]

How long do you want the campaign to last? Because it could easily start with just small-scale skirmishes with orcs weilding guns, and end with assaults on supply depots and research base (which is where the scematics are most likely placed) and finally the main camp, with the large stock of weapons to destroy.

Or it could be much smaller, only featuring one camp or two with the gunpowder and weapon designs, as those are the only orcs who have the weapons for now.

[nikolamat]

How long do you want the campaign to last? Because it could easily start with just small-scale skirmishes with orcs weilding guns, and end with assaults on supply depots and research base (which is where the scematics are most likely placed) and finally the main camp, with the large stock of weapons to destroy.

Or it could be much smaller, only featuring one camp or two with the gunpowder and weapon designs, as those are the only orcs who have the weapons for now.

Well I don't want it to get old so any ideas on how to keep it "fresh" while still having a few sessions revolving around this? I am thinking at first of following spies to their HQ inside korvosa then raiding that and looking for info and the like . and then have the PCs attack the camp with supplies. Don't have any ideas for content in between those two events. Any ideas would be appreciated. Also I am trying to keep it as little combat focused as possible. So far the PCs have captured one spy which they found in a cave near Korvosa.

[mig el pig]

Some possible minor elements which could fill a session or add's some flavour to the story:

-a 'scientist/alchemist' that was abducted to perfect the weapons.
The party could get involved with the investigation.

Possible twist: Perhaps he wasn't abducted after-all. He joined willingly because he wants revenge or was promised something.

-Shortage of Sulphur due to raids on caravans/mines and a peculiar merchant buying up large quantities

Possible twist: The merchant is purely in it for the money and is actually selling unpure sulphur which can cause malfunctions when firing the cannons/muskets.

-A noble is raising troops to eradicate a great band of marauders in the hills on his land. In truth both are one the side of the Magnimar.

Possible twist: His uncle/brother/father has a high position at court/army. Is he a part of the scheme or not?

[nikolamat]

Some possible minor elements which could fill a session or add's some flavour to the story:

-a 'scientist/alchemist' that was abducted to perfect the weapons.
The party could get involved with the investigation.

Possible twist: Perhaps he wasn't abducted after-all. He joined willingly because he wants revenge or was promised something.

-Shortage of Sulphur due to raids on caravans/mines and a peculiar merchant buying up large quantities

Possible twist: The merchant is purely in it for the money and is actually selling unpure sulphur which can cause malfunctions when firing the cannons/muskets.

-A noble is raising troops to eradicate a great band of marauders in the hills on his land. In truth both are one the side of the Magnimar.

Possible twist: His uncle/brother/father has a high position at court/army. Is he a part of the scheme or not?

I like the alchemist part, but what level should I make him? I have 3 guys at level 3 a druid a rouge and a sorcerer. What level should I make him so the battle is difficult but not result in a TPK?

[ReaderAt2046]

Another possible plot twist: There's a major conspiracy among the mages of the world to suppress knowledge of gunpowder, as it allows the ungifted to challenge the supremacy of mages. So just by finding out that gunpowder exists, the party has been marked for death and will be hounded by summons, mages, etc.

[TriForce]

altough its a bit different, you could look up the "Red Hand of Doom" campaign.
its about a invasion of hobgoblins and the PC's need to do all kinds of things to slow it down to give the defenders time to make preperations, after wich they need to help defend the city against the ( hopefully now weakened) horde.

its a bit bigger then what you are talking about, but it should give some good inspiration at least