So what is in this screen? This is the tower dialog screen, to open it you just click on a tower on the field. On the left side are your target selection and stay selection options.
Target selection allows to change what the tower shots at: closest alien to this tower, the closest alien to the exit, the strongest alien, the weakest alien or don't shot at all. Why none at all? Because some shots could heal an alien and you may not always want that.
Stay selection is when it decides to check the target selection. Until dead means once an alien is selected the tower fires at it until it is dead. Best selection means the tower will fire at least once at the selected alien and then determine if it is still the best target.
The icons on the top right are (from top to bottom) the weight of the tower, the max weight of the land it is on, the range of the tower and the damage dealt. If the weight of the tower is greater than the weight of the land max the tower will get damaged. The greater the difference of weight the greater the damage given over time.
The decommission button destroys the tower. You get the gunner back, but not the materials or crystals.
Finally, the crystal buttons at the bottom. This allows you to add crystals to the tower. The top number on each is the amount on the tower, the bottom number is how much you have. The crystals are based on contrasting colors on the color wheel. I use 8 colors so colors 4 places apart are opposite. If you add blue crystals you will heal blue aliens by the damage amount (heal by 4 in this case), if you add the contrasting color, orange, you will increase the damage dealt by the tower (to 8 in this towers case). And yes a mouse over show the color name, for those who have trouble seeing the specific colors. The colors are blue, cyan, green, yellow, orange, red, pink, purple.