how can I create a mini Maiden Castle in my garden?

[Hasakane]

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I've never done this, but imagine that if you turf over the mound, that'll make it rain-proof. Or maybe a centre of hard core, if the mound is going to be that big?

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You're welcome!

I've just had a lightbulb above the head moment! If you do some research into how Charles Jencks constructed the banks for the of Cosmic Speculation, that may show you how to do it!

That should be The Garden of Cosmic Speculation.

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that's such a cool idea.
You should bury someone something in it to confuse future archaeologists, too.