Tips and Tricks
Do not splurge all your cash defending one position - there's usually more to do. Leave a little in the pot early on.
Always read up on new turrets and enemies in between levels. That way you'll avoid the confusion in setting them up while fighting the enemy.
Don't neglect your base's unique abilities - they can be life savers. Value it and exploit it.
In case you've forgotten your sentinel can be moved don't leave it twiddling its metal thumbs.