How to build a good defense
Here we go, next to just being active, only a good defense let's you sleep at night.
But how do you do it?
Having only level 8 bases helps. Having the strongest towers
is great as well, but even this is not enough against a clever opponent.
Let's be frank, you will be destroyed, but it is a big difference if a weak opponent
cannot touch you and a strong opponent needs 4 or more attacks to destroy just one
of your planets.
1. Build bunkers and fill them with the best available units. Colossus or Falcons would be a good choice.
2. Group air and ground units together. Sniper Towers are your most flexible and toughest unit. 
3. Deploy your towers around your bunkers, enemy attacks target usually bunkers first and your towers have a great time with target practise. 
4. Build your houses, banks and silos around your base and your bunkers. This way many enemy umits will loot first and never reach your defenses. The lost coins are not important.
5. Use traps to destroy ground units and walls to channelize them and support your defenses.
6. Don't forget your stargates, group any leftover houses, silos, banks, starports, camps or
laboratories around them. A single shot by a Marine will kill all units in a star gate.
Finally, to,give you an idea what I'm talking about, I'll show you 2 examples of advanced base designs, which I liked. The strong defended planet:
