Using Google Maps, it would find routes to Tower Point, and the enemies would follow those routes to get to Tower Point, and you needed to build turrets to prevent them from destroying it.
The limitation that prevented me from polishing and then releasing it however was the number of requests that can be done through the Google Maps API per day. There probably is a way around it, such as caching or talking to Google about a higher limit if needed.
Train Defense works in a similar way, only you are constantly moving and needing to defend multiple carriages rather than a single point. Though I never got around to prototyping this idea, I would like to do it some day.
Talking in the pub with some friends about it, we thought it would be great if it could have a Facebook component, where you could capture areas of the world, and would need to defend against others taking over.