1st Modification:
For my first modidifcation, I would make it so in order for the teams to win, you'd have to have three balls in your circle. That way, it'd be a bit more of a harder and longer game.
2nd Modification:
My second suggestion would be to have four teams, four forts. That way, it would also be a bit more harder- having new apponents. Of course, you'd have to add a few more players as well. Maybe the teams could be teams of eight.
3rd Modification:
My last modification would be to make it so at least three people on your team would have to touch the ball before it goes to your fort.