The AI in this game can sometimes be smart enough to sidestep your offensive lines so it is crucial that you cover all your channels and leave no opening unprotected.
Landscape
Try and use the natural landscape to your advantage when your cash resourses are too low to be effective.
Spice
Although there are many features you can unlock in this game a good strategy is to focus spending your spice on your towers as these are the building blocks of any successful campaign.
