I think they covered it for the most part but to be clear on how these things work with the spawn. If you put down a work bench it creates a "protected" aria where things will not spawn within a certain radius of that work bench. If you place workbenches and ring the inside of the walls (gaped out based on the circles) it will prevent things from spawning inside the walls. The Raids are on a 46min timer, when it tics that timer there is an RNG mechanic that determines rather or not a raid will happen, then there is a "Raid Pool" that determines what will come. Some raids are determined by the next boss you have to kill. For instance if your next boss is the Elder your attacked by Grey Dwarfs, Bone Mass being next you are attacked by Draugrs and skellys. After killing Bone Mass Modder will send Dragons and so on. There is also a secondary pool that unlocks after you kill the bosses that could send non boss specific raids such as trolls or skeletons these are included in the same RNG roll as the boss pool so you will only ever have one type of raid and you wont have 2 raids stack on top of each-other. so youll never fight Drauger and Trolls at the same time. To deal with ground based attacks spikes help, or you can dig a trench outside your wall or build a dirt cliff. as far as the air based attacks Fire arrows will be your friend. like Lumber Jack said some enemies can walk though water or swim around the wall. It can be frustrating when a troll rinses your beautiful walls (happened last night to me) but its part of the game and despite the frustration it keeps it interesting. Good luck finding the solution that works for you and good hunting.