Tunnels can be built on objects
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Tunnels can be built on sources/minerals/controllers (I'd assume keeper lairs as well)
Not sure if this is intended or not.
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This is not intended, thank you for your report, we'll take a look.
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Actually, on the second thought, it's nothing wrong about being able to build tunnels under these objects (because you can't actually walk there). This is the same as that you can build a road under a non-walkable structure.