Before reading this post this post is meant for lvl 25-
Recently there has been a lot of discussion on the topic of what turrets to put where etc etc.
I provide you with my view on base defence. In the time I have been playing my base has been flattened once.
Okay lets start, look at your bridge this building should remain unharmed as much as possible as it stores resources. Furthermore storage modules (or in theory anything you find important) you would want to be protected. In this post I will use the term a block. A block has the size of a combat module and has length 1 by 1. The bridge then would be of length 2x2. First things first the setup I reccomend is your bridge in the center of your perimeter, evenly distribute your important buildings in the blocks which are adjecent to your bridge. Evenly distribute as much as possible the blocks adjecent to these blocks with combat modules and filler buildings(This way explosive weaponry only damage 1 combat module).
On the subject of weaponry:
Heavy beam lasers can be outranged with a variety of weaponry on longbow destroyers and rancor battleships, thus rendering them useless despite a very high dps. My recommendation for the turrets are for players which do not yet face longbows and rancors 4x heavy beam lasers on the combat modules, 5x heavy gauss cannon on the combat modules and bridge. For players which do face this threat my recommendation would be 4x heavy gauss cannon on the combat modules, 4x heavy plasma chargers on the combat modules and 1x heavy beam laser on the bridge. The heavy beam laser is to counter the so called blitzers which rush your base. The heavy gauss cannons function is to counter anything which outranges the heavy plasma chargers.
Now on the subject of armor and shields:
Since I assume you are lvl 25- your combat modules will most likely be lvl 6 or lower therefore only your bridge has a shield slot. The biggest threat against your bridge using this set up are rancor battleships with a plasma battery set up. Therefore I would recommend using a shockwave shield, my rule of thumb for armor versus better turrets is: The health of the turret should be greater or equal than 2x the health of the turret without any armor. This ensures your turrets live long enough and have a decent dps. Use the armor which gives you the most points such that the weight limit is not exceeded.
On the subject of the level of your combat modules:
when your base protection ends all your combat modules should be lvl 4 or higher and you should have researched heavy gauss cannons 1 and up to heavy beam lasers 3.
Before reaching lvl 15:
You should have researched tritanium up to lvl 2
Before reaching lvl 20:
You should have upgraded all your combat modules to lvl 5.
You should have researched heavy gauss cannon 2 and heavy plasma charger 1
You should have researched heavy beam laser 4.
Furthermore you should have researched tritanium 3 and shockwave shield 1.
Before reaching lvl 25:
You should have upgraded all your combat modules to lvl 6.
You should have researched heavy gauss cannon 3, heavy plasma charger 2 and heavy beam laser 5
You should have researched tritanium 4 and iridium 1.
The combat modules you should use are:
Heavy plasma charger 2
Heavy gauss cannon 2
Heavy beam laser 5
Tritanium 4 x2
Shockwave shield 1.
Any spelling mistakes are my bad and do provide your feedback.
I have added a picture of my base. This setup has beaten 6x rancor with a plasma battery set up. They never even got through the shield of my bridge.