>>96392895
Minimum wage for a mercenary warrior in 3.5e is 3sp, so 6sp for an adequate/generous wage.
Assuming 30 days -> 180sp or 18gp for a Soldier that will serve for about a month.
How much do you want to go against the garrison of a small town, 1000?
1000 x18 = 18,000gp.
The average treasure value for a Level 15 party is 22,000gp.
It's way beyond Level 9 for sure, but it's not far-fetched for a Level 15 party to be able to field and maintain 1000 men monthly, assuming they are not limited to just 1 adventure per month.
What legal ruling stops you from using said men to conquer something, or establish a mercenary company and get into some lord's good graces?
On a completely different note, a Castle as per the DMG costs 500,000gp.
What stops a high-end party from collectively buying such a building and make it a base of sorts?
I agree with you that there is no dedicated section on the core two books (DMG, PHB) on HOW to rule or establish a base/stronghold/whatever, but it does not strictly forbid it.