Here is something that I have been using for a while and is now very effective with the new machine guns. The basic bunker is a small bunker consisting of 7 blocks and can be build while the enemy is reloading. Build it using this diagram with the front pointed away from you. It will allow okay cover quickly.
Diagram: http://i1228.photobucket.com/albums/ee443/drewgon13/Blockbunker-basicform.png

I will post later directions on how to turn your basic bunker into a Normal bunker, an Advanced bunker, and so on until you have an entire fortress built.

behold
the correct type of structures that are to be built in a game of such
not pink fluffy cloud bridge castles

I like to make bunkers in mountains. making it is just mostly digging and making holes to shoot from.

But If I make a bunker on land. I usually make it multi-floored, pretty much like a house, but the bottom walls are double layered, to decrease the chances of the other team breaking it. I also put windows where we would need them, not just giant ones where you can get shot at easilly.

And I like to build trenches around my bunkers.

How I are make bunker?

Easy.

STEP 1: Press the "1" key.

STEP 2: Hold the left mouse button down, when your character is facing directly below him, until 2 blocks are destroyed.

STEP 3: Hop on in and enjoy.

By the way (1.) this is basic knowledge and (2.) It assumes you crouch all the time, which is a shitty strategy, and (3.) Machine guns will melt this in 4 seconds.