Arcane’s Tower Defense is a top notch 3D tower defense game for Windows Phones that very popular in the existing paid version (which is only $2). Now there’s a free version of the game that is the same except it has ads. So once you get really into the game and sick of the ads you can pony up and splurge. The free version has 5 stars across the board. Here’s a video to remind you what you’re missing:
The free version is actually updated with a new UI and has shared high scores. You can read more here and here’s the download link.