Will there be more added defences besides just a turret?
Well currently there is always the well placed and well timed explosive blasting cap thingy, which considering that raiders always spawn in the exact same place is very, very exploitable. (<- DEV HINT)
It's easy: After the first raider landing hit pause and memorize their landing location. When you're done fending off that first attack just spam as many blasting charges in that same spot as you can afford. When the next raider group lands, simply set them all off at once. For the third and subsequent raids it will be easier to remember the landing spot because it will be covered in nice carpet of blood and raider bits.
There will be a lot of everything added, defensive tools too :)
Turrets in general are somewhat a hot topic - due to their power. There was a notion from Tynan to remove them all-together, but community seems to like the idea of manned and less versatile turrets instead.
Since when RimWorld has became a war game? We only use turrets and charges because of the unlimited raiding parties. Focus on them, not on defense. Make the more small but intense fight instead, not throw a bunch of raiders at our face every single time.