All upgrades add cash.
This is an extremely well made and thought out TD game. HQ Guardians goes deep into the TD genre and gives us things that none before have. Think of command and conquer meets bloons td and then you are kinda in the right zone. Build building that will then create units/towers for you to use and deploy. They are not all static buy you even have some units that will fly around the screen hunting down the enemy.
Use the mouse to build units and building, then place them also using the mouse.