It depends on where your base is placed. You need to look at the terrain and figure out what the most likely method of attack will be. If you turret is pressed flat up against a mountain or hill, make it anti aircraft. I usually don't worry about the front of the base terrain. Build an anti vehicle and anti infantry turret up at the front. If one turret in the back is against a wall, build an AA turret. You want to do the opposite of the turret at the front. I'll try to make a little figure. A= AA guns, I = Anti Infantry and V= Anti Vehicle. ----= Wall or cliff.
V I If you are out in the open, it is a bit more tricky. Here's when you are against a wall:
-------------------- I use this most of the time. Just look at the terrain and decide what is best. If your base were an enemy base, how would you attack it? Put measures in place to prevent that from happening. As for defending an area, here is what I do: Depending on the terrain, I will usually have 4 Cobras, locked down and in a 4 point perimeter. Making a square shape almost. I like Wolverines with the Volley ability, but go with the tanks. Have 4 tanks guarding the Cobras. If you are playing as the Captain, build an Elephant and have it in the middle, also locked down. This way, you can make troops when you get low. If you are playing as a different person, do different things. Anders has a good flyer, and better upgrades. Forge has good units. Have units in the middle, to counter the infantry of the enemy. Make sure they are partly upgraded. With this strategy, you should be able to defend almost anything. Throw in some extra vehicles if you want. Wolverines are good for AA combat. I hope that helped you! If you want me to tell you the strategy I use when playing as the Covenant, just ask. I would be happy to tell! :)