These are some challenges for those locked inside with nothing to do. No mods allowed, and just a screenshot of the completion should be fine. This is for mid-late game players, so players who have just started may have some trouble.
Easy 1: Reverse Blitzkrieg: Defeat Germany as Poland (gens allowed) in 1939.
Normal 2: Pacific Island Pain: Win a conquest without any air nor missiles.
Hard 3: Napoleon's Dream: Control Barcelona, Moscow, London (let Germany take it), and all of mainland France, Germany, Belgium, Netherlands, Italy, and Switzerland in 1943 or 1939
Very Hard 4: Russian Swarm: Defeat Germany in 1943 by only producing infantry (air allowed).
I went overboard with this one 5: Mare Nostrum: Control all of France, Spain, Portugal, Switzerland, Algeria, Libya, Egypt, Morocco, Yugoslavia, Greece, Turkey, Allied Middle East (not Oman nor Yemen), Belgium, Netherlands, Bulgaria, Hungary, and control Cologne, Munich, Bucharest, London, and that one city next to London in 1939.
Give feedback (More!, You're crazy, help, etc.) if possible, and have fun!
