Since this game was meant to be a hybrid tower defense game, there should be an event where monsters are spawned at our entrance and we have to defend it.
My suggestion is, we should have an event where, when it starts, there’s a button we can click to defend our bases from incoming monsters that spawn at our base. We can’t create any units. We can only have our Kings together with any towers or barricades etc to defend the base. There can be many levels for the monster attack and with each level, the difficulty will increase, until you can no longer defend the base. The end result is to see how many monsters you can kill (just like a normal tower defense game is).
It can be similar to the existing war season, but instead of raiding other alliances, it is just a challenge as to the amount of monsters that an alliance can kill collectively. And instead of fiefdoms for tax reward, we can have a similar system for “food”, i.e. a reduction (percentage) of food consumed for raids. (The story is also consistent, where it can be explained that the monsters killed are cooked for food supply).
Furthermore, since there is only one instance where we defend our base until it is destroyed, it can be a mini event that only takes one day to complete. It can be incorporated in between war seasons and ninja events easily. It will definitely spice things up for the game.