Basically, you capture the tower by winning a guild spar against the defending guild. You wouldn't be able to actually kill other players in any other area but the spar arena. This would help in the Facebook server as many guilds can easily attack or defend with just one player due to the lack of players on the server. Let me know what you think.  

im not clear on how this would work.  

The defending guild would have to be constantly sparring, and that would be so horrible after a few minutes, but I think this idea can be worked on and pulled off very successfully.