The planters are 1/4" Corten steel plate. The upper wall was attached to an existing CMU retaining wall. The lower wall/planter is attached to a framed substructure. The plates were attached in sections so that they would be manageable for the construction crew. We used exposed fasteners (which can be seen at the lower right corner of the photo), but the sections could be attached with hidden clips, fasteners, or welds.

Here's a link to a previous discussion regarding Corten: https://www.houzz.com/discussions/love-this-look-but-how-are-you-keeping-the-steel-from-rusting-away-dsvw-vd~181816