In base or classic mod (most played) there are 4 types of turrets:
- big turret (changable barrel)
- bulid "in wall" turret
- landspike turret
- spider clamp turret
Ad. "big turret" (barrels) (non remote):
- mortar turret - long range, shitstorm, not as think useful turret, it can rape friendly base ~~ (useful if there no spieder clamps, landspikes and invs at its range)
- ELF - drain energy from nearby targets (useful agains cappers)
- plasma - shoots slow projectles at mid range (useful against all targets)
- AA turret - mid/long range turrets attacks "hot" targets (sucha as vehicles), uncheatable with flares (i think useless in classic mod)
- missle turret - fires missles at long range - very useful when there are more than 1 active missle turrets (very useful against vehicles and cappers "if there are 2 or 3 turrets this type)
Ad. "bulid in wall"
It's a very helpful turret - decent damage, and very accurate (static base asset)
Ad. Landspike turrets(remote):
Can be placed on ground, try to place them at open areas, where it has a big attack area, and can't be easly destroyed. It have a little splash damage. Again I think it have too slow projectle, so it is nerfed in classic.
Ad. Spider clamp turrets(remote):
Can be placed on solid surfaces (such as buildings). No splash damage, but fires accurate and fast projectles. Mainly useful as a flagstand defense agains cappers.
Mounting barrels to big turrets:
First of all, think against what it should work. Plasma and missle turrets are most useful I think. If turret attack range reach flagstand, rather use plasma, or ELF turret. If attack range can reach shit, use missle turrets.
Placing spider clamps:
Try to cover them from enemy fire, and try to let them attack a strategic area (eg flagstand). Also try to place them in places that let you place more turrets.
Placing landspikes:
imho best strategy is to make a "landspike field" or "landspike path", it mean to place many landspike in a strategic area (eg. in front, or behind flagstand), or place them on a path of incoming or evading capper. It also can be a good idea to place them around "strategic places".
I wrote here some basics, farming (placing turrets and stuff) sometimes is a complicated and hard work, but I love to see only my own turrets setting and its effectivity

Sorry, if this post is hard to uderstand. 05:20 AM here, I'm tired, drunk and stuff, hard to use non-native language now ;]