I think this has been discussed before but I couldn't find the thread. When one animal attacks, the very least information the attacker will get in the battle report is the condition of the target town's wall.
Sufficiently large numbers of animals attacking, maybe 20ish, will trigger the Catacombs and now the attacker has information about the target country's army.
If the battle lasts for more than one round, which it almost always does with groups of animal attackers, even with larger numbers of defence units vs attack units, the target town's towers attack and the enemy now has, at least, partial information about the target town's towers.
This happens despite the attacker being Despot and the target town being Collectivism, with her Ren and Industrial units, towers and walls defending, as well as my hero reinforcing with the equivalent number of animals as the attacker.