MC has the power to erect defensive structures with a certain level of range and power.
However, he can't defend all the time. He has a timer in each world, numerous goals and the option to lose a lot lest he fail to complete the bare minimum.
So, what can an archer tower do against an army of ninjas. Group of specters, ghosts and souls from the underworld.
Or perhaps those dragons, elves.
Will his shield attack hard enough?