Empty fortresses

[Zeldor]

He steals them first :P

[Agema]

Bribes would cost money - but it can come from an economic system outside the game's represenation. For instance, giving cooperative nobles ownership over the land taken from the unruly scum causing the unrest after you take the province; getting the job as your PD leader; being favoured supplier for the materials for your temple; etc. Makes no difference to the national tax, but that theoretical noble is richer.