Manor-Lords:Treasury
The Treasury is the money you get as lord of the Region. You can increase your Treasury by taxing your citizens through land or trade tax. This will convert some of the Regional Wealth to Treasury every month. You can adjust rates via the Manor.
Even though similar, it is not to be confused with Regional Wealth, which is the overall wealth of everyone in your territory. Think of it more like your own personal wealth that you can use for expanding your territory, hiring Mercenaries, or diplomacy.
You can see the size of your Treasury in the upper right corner of the screen. It's the number next to the coin stacks.
Treasury uses
Building new Settler's Camps
Once you've claimed a region, you can build a new Settler Camp for 250 Treasury.
Hiring mercenaries
Why let your band of loyal settlers run into battle when you can pay someone else to do it? Hiring mercenaries is a good use of your Treasury and is paid monthly. Rates vary depending on the type of group.
Increasing your Retinue size
The Retinue are like the Special Forces of Manor Lords, so increasing the size of your unit is another wise usage of your Treasury.