This espionage must be carried out every time before an attack.
But not if the target is:
- A Robber_Baron_Castle, in Great_Empire (grass map).
- A Barbarian_Tower or Fortress, in Everwinter_Glacier (snow map).
- A Desert_Tower or Fortress, in Burning_Sands (sand map).
- A Cultists_Tower or Dragon, Fire_Peaks (lava map).
This is because, if espionage is carried out once, for a target, of the above, then every target of the same level has exactly the same defense plan. For example, every Fortress in he same map has the same defense plan and every Robber Baron Castle, level 22, with 2 victories to level 23, has the same defense plan.
Avoiding useless espionage missions, helps you to spend less coins.
You can see spy reports for every target of the above categories here: