Arrogance

The only thing I would recommend to the author is to strengthen the garrisons separating the players; otherwise, there is no incentive to capture a neutral castle and develop it, as it is much easier to reach the neighbors and defeat them.
The AI does this; when I captured my neutral castle and went down to the lower neighbor, there were already battles raging on their territory. Moreover, all three opponents participated in this "celebration of life" (even though the neutral cities had not been captured by any of the AI). In this regard, I also didn't gain any significant advantages from the neutral city – there simply wasn't time not only to develop it but also to bring troops from the existing creature generators into the battles.