The secret to this one is destroying the obstacles as fast as possible. They are worth more money in this level than in others. You don't get glue, so make sure you put all your weapons in the best spots on the inside corners of the road. Also, you have to target the fast enemies that run ahead. The towers don't shoot at them right away on their own. It is possible to "perfect" this level (destroy everything), but it's hard! Good luck!