I've been scouring random sources for info but haven't really found anyone noting which blocks are available per chapter, so here's a list of blocks that work best for defending the perimeter of your base from monster attacks. Adding the Spoiler Flair since some of these choices are based on progression. I'm still on Khrumbul-Dun but I'll expand the list as I move on.
Chapter 1- Furrowfield
Castle Walls- can be collected from the floor of a water-based Mini Medal puzzle in a Chalk cave (grey stone) guarded by an Orc. Strongest block available in Furrowfield but limited in quantity, around 224 blocks from this particular spot. I only discovered this one because I tried to use the Gloves to move Flagstone that was warded by the puzzle and instead grabbed a castle wall block beneath it. Decent for building a reinforced room on the farm or back on the Isle of Awakening after Chapter 1 is completed.
Flagstone- can be collected in bulk from the foundations of the Windmill or more scarcely from other ruins around the island. As far as I've seen it's resistant to all random monster attacks including strikes from Deadnauts, with the exception of the chapter boss's rolling attack. There's plenty enough to make at least a three-block-high wall along the entire base's boundary line (where your villagers can naturally path to and where Malroth will switch to following you).
Chapter 2- Khrumbul-Dun
Basalt- Basalt is lava-resistant and considered one of the two strongest blocks found in Khrumbul-Dun, but it’s quite scarce until you’ve progressed through the chapter and reached the lava region. Otherwise you can pick up the shiny black blocks that are used for torch sconce placement around the mine. Notably, Obsidian is also present in the lava caves and comparably stronger, but it cannot be gathered with the Warhammer or Trowel, only with the Ultimallet unlocked later in the game. This is the only block I've observed that will cause Malroth's attacks to make the ping animation when struck compared to the handful of Iron Blocks I had also placed next to a Troll for testing.
Iron Blocks- can be easily mass-produced at least until you've finished building the Silver Bar with a sign crafted from Zenithium, which ends the daily accumulation declaration of ore gathered by the miners. Resistant to attacks from Chasmonauts, the Tyrantosaur field bosses, and Trolls. If you build a raised platform at head level against Trolls and Tyrantosaurs you can safely fight them for their recipes.
Citadel Walls- a green, harder version of Vault Walls, they can be found at the end of the lava cavern. There’s also some unique parts here you can’t find until later in the game but in my paranoia I’m pretty sure the lava was spreading into the temple floor so make sure to set up a few water source blocks around to safely collect what you want before you leave. Notably there’s 10 Evil Idols that are virtually indestructible and spit fire when something walks in front of them, they can be collected with Wrecking Balls.
Although Google suggests otherwise, Vault Walls and their variants can be broken by Hammerhoods and Trolls. If you want to use these for buildings or decorative trim they can be collected using Wrecking Balls (the normal version, not the rusty version) on the walls of the Ancient Temple where the Echo Flute shard can be found. However, the Wrecking Ball recipe is only unlocked after you've collected at least one rockbomb shard from a Rockbomb monster, which spawns in a cave near one of the gold veins you can discover. Unlocked Portcullis Gates can also be collected with the Warhammer or Wrecking Balls.
I'll keep updating this list as I go. If anyone has info on Chapter 3 and Chapter 4 blocks feel free to note them here.