Defend Yourself Strategically in Carveola Files

How does the game Defend Yourself Strategically in Carveola Files work?

The zombies are invading the whole northern area of the country and are starting to head south, where all the refuges are full of civilians. Stop this from happening by sending all the possible soldiers and war machines to strategically raise a barricade and defend yourselves from all the waves. Don't let any of them in!