Some way to counter spymaster actions in your cities that doesn't require you walk armies around none stop or building a frankly stupid amount of garrisons, may that be using the spymaster in a count spy mode thus looking for other spies or a building like that works like the SIS or NSA. Say one territory that give nothing but increased detection as well as a chance for spy actions to fail and add grievances or kill the spy which ever possible in the engine