Click on the tower to select it and information about that tower will appear. Once you have selected a tower, move the cursor back onto the map. The green box shows where the tower will be placed, if it goes red you cannot place a tower there. Place a few more towers onto the map so that the aliens will have to move around them. You are now ready to defend agaisnt the first level of aliens, so press the start button. Towers can be upgraded to increase their power.