So, the "correct" way to play is actually to focus entirely on canon towers. Get the tower upgrade upgrade, but don't get the second "more towers" upgrade.
Then focus entirely on making canons on the left side facing right and canons on the right side facing left. Do not spread them out, put them all in a single line. Not vertically. Horizontally. What you end up with is a line of death with giant balls crossing each other in the middle. It's just a constant stream of DPS. No combo towers needed.
When you don't get the option of canon towers, get poison towers as a means of improving the damage of your canon towers.
All you need is as many canon towers as possible shooting at each other in a single horizontal line. A giant never-ending stream of cannonballs.
This is because projectiles pierce enemies, multiplying their damage for each enemy hit effectively.