The build and placeable Gong how about making it a clickable item like the potions
You can build it, place it but there will be no enemy spawn, only after using to gong(with sound ‘yay’) the enemy will spawn.
The gong can be used as a decoration.
You do not have to place it everytime anew to trigger the attack.
The attack can be triggered when you are fully prepared and you can be sure that the attack does not occure at the same time as the daily attack.