Japanese Home Islands defences added to invasion risk

What shore defences did Japan have on the Home Islands during the Second World War?

Quite extensive defences, which added to the difficulty of any invasion.

Japan had substantial shore defences across the Home Islands. The answer does not identify particular guns, fortifications, or garrisons. It instead adds those defences to the problems facing any invasion. Open this moment for the strategic conclusion, while recognizing that the detailed defensive system is outside the answer.