The entire world except for one city have been conquered

It has been a year since the outworld humans invaded the Pokémon, and since then, most Pokémon and their settlements have already fallen under the iron fist of humanity. The only place that still stands is the biggest city of the Pokémon known as Poketropolis, it has managed to keep itself protected thanks to a forcefield around the city, which the humans haven't figure out yet how to get through yet, the forcefield is kept together by a rotating group of pokemons working together to maintain a city wide barrier, if at some point not enough pokemon are maintaining it their city would be left defenseless from the human invaders.