I challenge you!

What about the central garrisons? They grow very quickly. I had no chance because the central garrison remained with the enemy, and it already had a million imps and demons.

20%-40% increase in 1.2.1, not 100%, as in 1.0, for example.
You probably played a lower hardcore version, but even there, if you capture the central tower (it is not captured through the central outposts, but through the side ones - it takes longer, but there are fewer losses), you can achieve a very significant increase in the castle. (in 1.2.1, the increase from the central one is also reduced in the castle)

At what point was it impossible to defeat the garrison (month and week)? What was your chosen difficulty level?