What about add an Spy unit, with low HP, LOS like a vil, speed lil more than a vil, no attack, but it has some special abilities:
-It is invisible from almost all units except “defensive” ones (I though skirms, pikes and scouts, and another spy).
-When a enemy building is in its LOS you can see what the building has garrisoned , producing or researching.
-This isn`t very special, but you could stay alert when the enemy unit cross a door and keep it open for your rush
It would be available from Feudal in TC and would cost idk 50F 50G, and TT 35 sec.
Some civs could have bonus or UTs refer to this unit.