You keep the outer perimeter at 4‑block wall thickness, but drop the middle layer to just 2‑block. Use 3×3 “t‑shape” modules so only 9 slots per layer, then stack three of those to get 27 slots total – that’s a solid wall and still gives you room for turrets. Add a 1‑block wide resource conveyor beneath the first layer so the gold flows straight to your store without wasting space. Don’t forget to place a gate on the inner side of each wall face, otherwise you’ll end up with a free‑for‑all wall‑hack.

You’re right, a single 2‑block wall on the inside is more decoration than defense. Double‑layer the outer edge—two 4‑block walls back‑to‑back with a 1‑block air gap for heat dissipation. Put a pressure sensor on the inner side of that gap; when a spike lands, it fires a 3‑turret line that sweeps the interior. That way you get redundancy without wasting the whole block count, and you can keep your gold in a 2×3 bunker instead of a floating marble ruin.

Good point—sensors can lag. Why not make the inner wall a 3‑block wall instead of relying on a trigger? Build a 3‑block layer behind the double outer walls and place a 2‑turret pair directly inside it. That gives you a passive defense that never needs a sensor. The 3‑block wall will still be thin enough to keep gold flow but thick enough to stop a spike. And the turrets will fire automatically whenever something hits the 3‑block, no manual trigger required.

Add a 1‑block “buffer wall” on the far side of the 3‑block layer, angled at 45° to the spike path. That way any spike that slips past the first two layers will hit the buffer and lose momentum before it can damage anything behind it. Keep the buffer at 2‑block height so it doesn’t clog your slots, but thick enough to stop a single hit. That turns the whole thing into a fail‑safe system, no single point of failure.