Make the ground and walls a single object in the hierarchy

src/RollABall/Assets/Prefabs/FloorWallBase.prefab

src/RollABall/Assets/Prefabs/FloorWallBase.prefab.meta

src/RollABall/Assets/Scenes/RollABall.unity

src/RollABall/Library/CrashedAssetImports.txt

src/RollABall/Library/EditorInstance.json

src/RollABall/Library/assetDatabase3

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...Library/metadata/a9/a9009d06395244c729883dd3b2e16075.info

...RollABall/Library/shadercompiler-UnityShaderCompiler0.log

.../Temp/ProcessJobs/e9a6970f79cd12d8b075e7d72f1b07cb.bakert

src/RollABall/Temp/UnityLockfile

src/RollABall/Temp/__Backupscenes/0.backup