Flak is the starting hero. He can be upgraded by spending gold. He only has only one upgrade path. Keep him upgraded when possible.
Laser towers
Laser towers work especially well against enemies that are shielded.
Layout
Build infantry towers next to attack towers. Use infantry towers to get the attention of your enemy and keep them in range of your attack towers. Infantry that are killed in battle will respawn over time.
Tesla Coil
You will have a pre-built Tesla Coil in one of the tower locations of every level. It can be used three times in that level to shoot a long range lightning strike that instantly destroys units near where it was targeted. Save it for emergency situations.
Upgrades
Unlock the MK II versions of all of your towers as soon as possible. You must purchase one basic upgrade in a tower line in order to buy the MK II upgrade.