block thickness doesn't really matter creepers will still blow it up and gang up with other zombies and skelatons and spiders etc. ground homes bases are not safe, i would suggest though of putting a taller outer wall two blocks beyond that wall, so there is one block between the two walls, and also as well as creating a safe and clever way of getting out of your base like an underground passageway with door and pressure plates or a tree semi close to the wall and climbing the tree using the longsword to leap into your base, going back to ground homes are not safe, creepers will go up to the edge of your walls and blow them up even with you not there and gang up with other mobs almost like team work, i had a completely sound base of roguelike dungeon no holes all filled up, completely safe and a horde of zombies came up the stairs and murdered me creeper blowed up hole in the bottom of my base/tower and zombies swarmed the hole it was nighttime and used teamwork to kill me. that wont happen with a second wall beyond your outer wall one block beyond that and as well as leaves on top or punji sticks, and if you want doubly protection i see a oil pool on your minimap you could take that and make a oil moat around your wall and punji sticks to kill them for good, but as i said ground bases are not safe and i would recommend a sky base to take out all of the trouble but as you have a base do that for now and use wall materials that are anti-greif i dont know a list but common sense things like planks, cobblestone and other manmade indigenous block are a nono, dirt isnt griefable but blows up really easily.
as for spawning with a ground base the light level goes to zero right outside the wall and a zombie might try to spawn a block outside the wall and try to get in right away and suffocate in the wall. try a roof or if no want one for the crops build a little floating crop area with a staircase, but in general ground bases are not safe and theres even a chance of the zombie spawning on top of the leaf top wall, because even if theres one spot for a mob to spawn in it, it WILL spawn and going back but ground bases are just not safe, this modpack is rediculously hard and making a simple 25 high block wall 3 feet thick with simple defenses is too easy, also a thought if its permanent being on ground you are attracting mobs from outside of your house and in the air and also mobs in the ground, and if you keep the base and make it permanent you may want to dig down a little bit and in to expand and although attracting mobs down below might sound rediculous there are tons of roguelike dungeons spawners and other caves but mostly roguelike dungeons catacombed throughout the entire map and the earth, one tower covers 2-3-4 biomes of underground catacombed stretches of dungeon and with spawners and dark light levels, you have a fairly good chance of hitting one by digging down in any general area and mobs will scent you and hear you and whatnot from a rediculously long way away, so really its either a sky base up in the air, or go down, but not too much deep because of gas, better of the two definately is sky base but even a cave base is probobly better than a ground home.
ps. sorry for the lengthy text but that is probobly everything you need to know about how to build a house in blood n bones, or at least the general understanding. there is probobly more problems or solutions and i dont know everything but this is everything i know about bases and theres a lot more into this than that considering i haven't really got into super late game scenarios and long term such as the end or full vulcanite armor and tools with magical crops mega farm or a animal/plant farm manopoly but this will serve you okay at least maybe for a little bit, and maybe will prevent any "WTF" moments, and every mistake in this modpack can be learned from, this isn't tedious, this isn't impossible, its just hard.
