Great idea, unfortunately once you set up defense early game, there is no way to lose... you just slowly expand and win...zzzzzzz. And full version is $10.
Build one archer tower. Level up to 6. Repeat until you have three Level 6 archer towers. Then build one archer tower as close to the start as possible and upgrade that one to corrosion.
Gets a bit tedious in later levels, removing spawners would help a lot and give it a much faster feel, rather than a walk backwards while shooting feel...