In addition to Defense Platforms & Turrets, you can use units in your Army to defend your base from invaders. The Barracks, War Factory and Helipad can each assign units as Defenders. Highlight one of these buildings and tap on the Defenders button. If this is the first time you have opened this feature, you will need to unlock the Defenders option before you can add units.
Defenders will save your base more often than turrets. Often when scanning bases to kill I will check for defenders and check what type. My first consideration before looking at turret placement is "Can I hit 1 or 2 of the defender buildings easily?". Your base will get passed by more often if your defender buildings are well placed. You also want to consider the timing on your defenders.
When you add units as Defenders, you’ll see a time value over each slot. This is the amount of time it will take for all units in that slot to deploy to the battlefield. The more units you add to a single slot, the longer they will take to deploy. A single Rhino takes 16 seconds to deploy but two Rhinos take 33 seconds. If you want multiple units to deploy faster, spread them across the the unit slots.
Timing defenders is just as important as what you put in your buildings. Ideally you have all three buildings release defenders at or near the same time. This allows the units to support each other. It's much better to have your mortarmen come out just behind some tanks from your war factory than all by themselves. Take the time to look at the timing of units coming out of your base.
After units have been added as your Defenders, return to your base and view the building. You should now see the selected units represented just outside the structure. This is also how you can identify enemy Defenders when you attack a base.
Finally try to use your defender units to balance your defenses. If your turrets are primarily there for anti air consider using mostly anti ground defender units. If mostly focused on anti ground for turrets consider anti air units.