What are some good opening strategies for Robo Defense?

Long Paths

Instead of running straight towards the exit have them run back and forth between your guns. This means that in the beginning you should favor "many guns" over "good guns" until you have at least some parcour for them to cross.

Strong Points

Check the ranges of your cannons. Those that encompass the largest area on your pathway should receive the first and the most upgrades.

But don't focus on just one such strong point. Have at least two with sufficient distance so that unit which slip through the first by moving in the shadow of larger units can be addressed later in their journey.

Good Combinations

Cannons with splash or area damage should be placed where many enemies will be at together. This means either at the beginning where the herd is still large or at places where units are slowed down and thus closer together.

Anti Air

Always consider that you need some anti air defence later on. Don't rely on a strong frontline. It's better to have them under fire the whole way.

my basic strategy is 2 or 3 guns (the blue ones) that force the enemy up. Then add a rocket or two up. Use more guns to force them back down to the middle, and then put some rockets in the middle to counter the eventual air ones, then I'll throw a slow one behind the rockets to help them out.

so... let's see if this works... W=wall, S=Start, G= Gun, R=Rocket, S=Slow