When it comes to defending your base, there are many ways to set it up. One is to surround your entire base (or at least your important buildings) with one layer of walls. By doing this, you can make your walls thicker than you otherwise would be able to, layerwise, and you can fit more buildings inside your walls. However, once your walls get breached by opposing troops, everything inside will be pretty much done for.
A better way to go about it is by building a “pocket” style base, where each important building is circled with its own layer of walls, with your important buildings being put as close together as possible to still save wall space. Considering the programming of generalist troops, this will really screw up opposing players. The hybrid method is also doable, which is where walls are layered in a method that is somewhere between the two listed.
You have a maximum number of turrets regardless of what kind you choose to build, so at any time if you wish to change turrets to something more useful or powerful, tap on a turret and switch it over. Mortar turrets are especially recommended if you often get hit by zerg rushes, where massive amounts of one generalist troop (such as stormtroopers) or scout troops (loot hogs) are attacking you at the same time.
Your important buildings, the ones that you want to guard, consist of your headquarters, as well as your resource buildings. However, change your strategy depending on your goal. If you’re trying to build a high number of a specific resource, then stop it from getting stolen by sticking all of your buildings for that resource (such as alloy depot and alloy mines) inside of your walls until you have enough to build what you need to build.
Never neglect your research facility as that’s one of your most important tools for battle. Research your troops to as high of a level as you can based on how often you use them. A high-level low-tier generalist (such as a high level stormtrooper) is nearly unstoppable in high numbers for all but the most advanced players, or players with the highest-level walls.
High level walls are very expensive, but work towards upgrading your walls to as high of a level as possible to stifle anybody who isn’t using a wall breaker troop extensively (such as a Bantha rider) and even to stifle the riders, holding them up for longer periods of time so that they can get shot up by your turrets. He who has the highest level walls, typically wins the battle.