It's escalating and it's already round 11... you shouldn't bother with defending your bonus zones, instead put all your troops in a couple strategically placed stacks, so that you can eliminate an opponent when he has 4 or 5 cards. You get your opponents cards, get to cash them midturn, then you use the troops you get to kill another player, get his cards, etc. That's how it works.
Well just look at green's last turn. He eliminated you, took your cards, cashed them, then eliminated yellow, took his cards, cashed them and then eliminated blue... that's what you should be doing in your next escalating game.