The half rounds are individual long pieces, get your self a circular saw (B&Q - £17) makes cutting them simple and quick. Start with the top piece and get it level as the posts amy not be 100% in line, then work down.
I am looking into terracing my own backyard, and came across your post. That is some amazing work you've done. Can you show us more photos, especially the ones in progress? I'm completely novice and I think in-progress photos will help.
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