Defending against trolls while they can be difficult is not too difficult.

First Islands... well Trolls will walk through a lot of water so unless it is of shore and you have cleared any possible spawn point you can still get attacked. So it isn't a "solution" really.

Defense a ditch of any height with a sharp edge they can't walk down. The longest reaching troll attack is about 8m-12m (not including rocks being thrown)

So I generally use a simple Ditch, Lip, Earthwork approach. A ditch that is at least 2m (usually 4m) wide and generally only about 2-4m deep. Then a 2m lip, and a Earth work that is 3-4m high minimum and about 2m thick. Then on the back side of that I put my palisades. This is almost always out side of all but a rock through attack. This means if they stay on the other side of the ditch you can't be hurt. If they get into the ditch having the palisade where it is and supported by a earth work means it is unlike to be broken. If you leave the space under the Palisaide empty (or for wood and rock storage) then even if they get in the ditch they can't hit any of your structures inside. (I can confirm this fully as they did get in and did no damage to any of my structures nor the palisades and crenellations

I also will line my earth works with 45 degree beams but that is much more a facade. However it gives the Trolls a target. They want to find something to target and attack. So if they have all the 45 degree beams on the outside of your walls to target they are much less likely to try to find a weak point. My first base missed this and they were able to get close enough to my garden to attack it through the wall and lost a full crop.

Is it over kill... yes... Will it defend against drakes... Likely not.

Aftermath of a troll attack

https://steamcommunity.com/sharedfiles/filedetails/?id=2631336648

Ring fort
https://steamcommunity.com/sharedfiles/filedetails/?id=2624190810

Wall Construction (serious over kill)

https://steamcommunity.com/sharedfiles/filedetails/?id=2616553274
https://steamcommunity.com/sharedfiles/filedetails/?id=2621478941
https://steamcommunity.com/sharedfiles/filedetails/?id=2616553824
https://steamcommunity.com/sharedfiles/filedetails/?id=2621479044

Complete forts.. of different sizes. (all built single player legit)
https://steamcommunity.com/sharedfiles/filedetails/?id=2616552654
https://steamcommunity.com/sharedfiles/filedetails/?id=2616552785
https://steamcommunity.com/sharedfiles/filedetails/?id=2624190722