Build a SMALL circular medieval tow...

Build a SMALL circular medieval tower. TOTAL outer diameter: EXACTLY 18 blocks (wall to wall). NEVER exceed 18. Wall thickness: 1 block only. Interior must be 100% empty (no floors, stairs, slabs, ladders, platforms, or decorations). This is ONLY a shell to cover an XP farm. ALLOWED BLOCKS ONLY: cobblestone, stone, stone bricks, oak logs, oak planks. FORBIDDEN: ores, bedrock, deepslate, moss/mossy blocks, cracked variants, decorative, rare, creative-only or modded blocks. If a block is not allowed, replace it with cobblestone. Do NOT scale up. Do NOT add detail for aesthetics. Keep it simple.