The real question is how much omniscient are each player is on his/her land.
We can divide the spy job onto 2 categories :
- Intelligence gathering :
- Passive form you discuss with foreign marchants about their contry and cities or you send someone travelers into that contry (tourist mode on)
- Active form you pay for the information with bribes or you try to infiltrate enemies a casern or research facility.
The kind & the quality of the information will differ between the passive & active form.
To know that a city has a big funace or that they a lot of soldiers garnisoned is the kind of passive information that can be gathered just by opening eyes & hears wide
To know what kind of research or the exacted quality of the force gathered in city will require the active form of information gathering.
- Sabotage, destabilisation, assassination.. :
- Any form of active destruction of enemies assets that can be from the destruction of a facility or the assassination of a hero to the poisoning of a whole city.
- Any form of destabilisation & subversion that can from spreading FUD about an ennemie leader in a third party city to sour the relation between those two or the bribery of an enemie hero or unit to the incitation of revolt in a city.
As in any good spy game counter spy can stop those activity from speading in your teritories.
Another point is the use of hero spy VS use of grunt spy/assassin.
Heroes must be free to travel between countries not at war but more famous is the hero and harder it is for him or her to not get noticed from the other side save if he/she has some steath ability.
The spy UI must be rather easy to use you just have to alocate money & grunts for the different tasks like in galciv 2.
Grunts spies can be affected to a country or to a city changing the scope & quality if the gathered information.
Grunts spies are in the passive information gathering mode by default you must pay for any other form of action.
Counter-spies not only try to twart ennemie spies move but also gather passive information as they spend a lot of time with your foreign residents, marchant, travelers...