There was an upgrade published last night (in Australia) for additional downloadable levels for GeoDefense Swarm. The first level, "Crossed Wires" is free and you can buy another 10 levels (5 medium and 5 hard) for about $1.
The free level is fairly straight forward. If you get stuck a solution is shown in the attached picture. There is probably also a solution using just thumpers. Start by building towers straight up and down from the exit points and then gradually build the path as you get more cash. Eventually you will run out of spots for additional towers, so start upgrading.