When Small Castle (This is a action card) was at the top of the Castle's pile,
I bought Ferry and tried to move -$2 cost token to Castle pile. But I couldn't it.
Number of game and server information is "Joining game #1967134 on frankfurt".
I'm sorry if this behavior is not bug.
It's not a bug because the Castle pile is not an Action Supply pile (its pile types are Victory and Castle).
According to the wiki the pile is only considered a Victory-Castle pile due to the randomizer being used to determine the pile type:
One Kingdom Card pile in Empires has 8 different cards - the Castles. They behave similarly to the other split piles; only the top card can be bought or gained, they go in order by cost with Humble Castle on top, and so on. This pile is treated as a Victory - Castle pile, as per the Randomizer, despite the top card being a Treasure - Victory - Castle. In 2-player games, use one of each Castle; with more players, use all 12 cards.
I am sorry for my misunderstanding, and thanks for explanation!
I understood this behavior.