Follow the water beyond the wall
Terraces are easy to recognize from a distance. Their important work is harder to see. USDA's Natural Resources Conservation Service defines a terrace as an embankment or ridge-and-channel arrangement across a slope. Its purposes include reducing erosion, trapping sediment, managing runoff and conserving soil moisture.
The 2023 national standard treats adequate outlets as essential. Water leaving a terrace must reach a location where it will not cause damage. Spacing, capacity and soil conditions belong to the same system; a well-shaped ridge cannot compensate for every failure downstream.
Sources: s1
Maintenance is evidence of continued use
The standard includes inspection after significant runoff, removal of accumulated sediment and repair of damaged components. A terrace is therefore a continuing relationship between people, soil and water. The national document is a conceptual source here; NRCS directs actual planning and installation to local technical guidance.
A better question for an old landscape
An archaeological hillside invites practical questions: where did water enter, where did it leave, and which features were maintained together? The surviving structures and later reconstructions need separate descriptions. A modern standard can suggest questions, but it cannot identify the builders or date a wall.
For the fictional islands, terraces offer believable work for a community: clearing an inlet after rain, moving trapped sediment or negotiating an outlet across a boundary. Those are proposed story details. They should remain visibly separate from a claim that a particular ancient society used the exact modern system described here.