Existential doubts that torment me (from times to times) and that I leave for reflection and comment if you so choose. Normal situations in the game, I think, but that may be the subject of different interpretations by different people or in different time frames.

Theoretical hypothesis 1:
Player AA has 50 castles. He is at war and this does not go well with him or he has other matters that do not allow him to fight it. As a way to reduce the pressure he demolishes 19 castles and gives 30 to BB, friend. BB is not at war, has agreements, or cannot be attacked for any other reason. AA's Main is now too defended to make sense to attack.
Solved the problem, the player resumes the 30 castles. In the owner passages there was no loss of troops.
Did player AA benefit from this transaction? He wasted no time or troops defending his kingdom. He did not lose castles to the opponent.
Did the BB player benefit from this transaction? He managed to produce troops without having to evolve one (several) castles or conquer them with loss of units.
It seems to me to be a good asset management strategy. Everyone wins. Everyone? Everyone but those who were fighting a war and winning. The CC player who has not been referred to before.
Wasn't the CC player harmed? And the player DD (another unknown until now) who had a strategy of conquering, or destroying, the kingdom of BB and who saw the conditions changed?

Theoretical hypothesis 2:
The AA Alliance has a new, small or war-depleted member, AA player, who needs to grow, evolve. All members send resources to the said player's castles. The player grows very fast.
It seems to me to be a good alliance strategy. It doesn't cost a lot to each member and makes a huge difference to the weakest AA player. Nobody loses and the AA player benefits a lot.
Nobody loses? What about the BB player who will be attacked by the AA player over time? This player had no help or was not quick enough.

Thank you for your contribution. With an open debate, new perspectives are achieved, a better understanding of the present sensitivities and the correct interpretation of laws and rules (there is the letter and the spirit). I don't want to make mistakes or illegalities, but I also don't want to fail to take advantage of legitimate opportunities.

1) I agree with what you say. It looks good and legitimate to me. But it is just a point of view with which we agree.
And this possible perspective:
The military grade of the BB player is unknown - depending on this grade there are many variants. The BB player did not lose any units to conquer the castles and started to produce troops for him. An advantage, 20 new castles fully developed to produce troops at no cost.
The castles turned yellow and available to everyone. But do you (anyone) want problems with BB knowing that the castles were passed? A "normal" yellow castle has a different period of reconstruction and defense than a "ceded" yellow one. Along with the conquest "attacks" follow the reinforcement troops - no one will send them back. Incidentally, the AA player now has many troops to support - or not.
Player EE, that brave and fearless stranger, will be able to attack BB. But will he want to? After all it depends on who the BB is.
Perhaps the basic question is the identification of AA, BB, CC, ...
2) I totally agree. That's what good alliances are for.
BUT the player was helped, pushed to grow. Perhaps even with the provision of some strong castles for the creation of troops. Castles that will be yellow in the beginning, of course, but we entered point 1 again.