I see nothing fair and reasonable in limiting the use of one of the troops. Some troops have some hidden talents. And with spies one of them is that 1 well-placed spy can sometimes come with just as much valuable information as an entire army of attacking spies.
Finding out the hidden values of troops is a great thing in this game.
Having more knowledge of the game than your opponent is a fair advantage
If your opponents can spend more time in the game sending spies to your castles, then you can send back, is a fair advantage for the opponents.
But a player that sends for example, one light infantry to an enemies castle as support to collect a battle report of every troop in it is the game functioning as it should be? It's not even a spy unit.
I will correct you on your one statement highlighted above. When using the spy trick the only information that you can get is how many troops are in the defending castle, it doesn't tell you research levels, building levels, construction sites, resource amounts, etc. so there is no way that they can give you as much valuable information as a successful attacking army of spies.
You're saying spies when any unit's capable of being used for the spy trick. If you were sending actual spy attacks against defending spies it'd be a different story because you need to actually win to get a spy report instead of losing the battle to get one like you do with the spy trick. Does anyone really believe that that's how the mechanics of this game were meant to function? Do we really get spy reports when our spies can't successfully win a battle? No.
Lord Dragoon: to be honest, it is linked to the fact that those who send troops in supports in friendly castles can thus receive reports of battles
but some variables make the strategy game much more exciting
Do you really want to prevent players from helping themselves with support? if this yes, I consider this unfair
btw, sorry for poor english translation
My suggestion above doesn't suggest making it so that your alliance and its allies can't support you, it actually states that they're the only ones that would be able to with the way the game currently works. It would still allow your guild and its allies to see your battle reports but it wouldn't allow for an enemy guild to send a single troop to your castle and have it attacked by someone else to get a battle report revealing the troops you have their, it'd actually make your enemies have to win battles with their spies to see what you have in your castle.
I'm not stopping players from supporting themselves or from letting their alliance or allies from stopping them, my suggestion stops enemies from sending support troops to their enemies castle to get battle reports revealing how many troops are there whether they win or lose the battle. If makes an enemy alliance or player win a successful spy attack in order to see how many armies are in the castle.