Merge Cannon

Chickens march at your castle in waves and cannons on your side shoot them. The cannons are also merge pieces: two of the same kind combine into a stronger one, which means every slot on your wall is doing two jobs at once.

That is the tension the game runs on. A wall full of small cannons puts out more shots right now, while merging them trades a moment of reduced firepower for something that carries you through later waves. Merging during a wave is how defences collapse; merging between them is the whole plan.

The castle itself takes upgrades separately, and those are the safety net for when the merging goes wrong. A tougher castle survives a wave that leaked, which buys you the turn you needed to finish a merge you started too late.

Each run of waves ends with a boss chicken, and it is not something a wall of small cannons gets through. Arriving at it with a merged, upgraded line is the point of everything before it, which is what stops the merging from feeling like busywork.