What is a brief resource you would recommend for getting a high level understanding of string theory?

I am just trying to get an overview of the basics, so the shorter the better. Although I am a mathematician I am not unfamiliar with the relevant physics prerequisites, I understand general relativity well and understand the main ideas of quantum field theory. Notions like compactification, world sheets and D-branes are fairly easy to grasp but that is where my current understanding ends.