At the start of a mission, before you have placed any towers, you can right-click to begin the mission instead of placing a tower like the prompt in the Tactical Recon bar says to. This allows you to accrue slightly more interest without having to waste some Resources building a tower.

If you hover over an empty space that you can build a tower on, you can press the following number keys (located on the top row of the character keys) to build the tower without having to use the mouse:

Additionally, if you hover over a space that already has a tower on it, you can press the following keys to upgrade or sell the tower:

Notes:

• These keyboard shortcuts cannot be performed with your keyboard's numeric keypad (which is the 3x3 cluster of numbers on the right side, if applicable).
  
• If you have the tower context menu open (to build a tower, or upgrade or sell it), you have to close it. The keyboard shortcuts only work if the tower context menu isn't open.
  
• These number key shortcuts are always associated to the listed tower even if you do not have access to certain towers during a mission.