I played something like this a while ago, with mixing bounds and defences. It wasn't very good, mainly because the bound was too easy, and the defence was screwed up. If you think youcan pull of the map though, go for it!

And my thoughts on what could improve it. Give the players lives for bounders, and if they get a civilian they could trade it for 5 lives or something. Also set it to have 3 levels of difficulty, I have yet to see that in a defence. You don't have to do these, but I think they would be cool.