Ground defenses are a good way of deterring the smaller attacker and those attacking for the resources you’ve gained over night etc, but defenses will not protect you completely, many high ranking players or groups of players will take great pride in smashing through a turtle, and if the prize warrants the losses the attacker will sustain, then they will attack.
Example of large Group Attack Vs Defenses, … c9bbbd2768

Ground defenses should not be built unless they are protected by antis, build a level 2 missile silo to unlock the Anti Ballistic Missile, each level of the silo after level 2 will increase your max antis by 10.

Once you’ve got antis, then your defenses have a least a small amount of protection, but antis are not unbeatable, so you will need to keep the number of antis proportionate to the amount of defenses you have.

A member of Obsidian* did quite a bit of research into the best ratio for defenses, and after many simulations came out with this ratio

    Missile Battery * 50
    Laser Cannon * 50
    Pulse Cannon * 5
    Particle Cannon * 5
    Gauss Cannon * 4
    Large Decoy
    Plasma Cannon * 1

* Thanks to JSJ for providing this, and thanks to Baron Von Plasma Turret for doing the work.