And David lived in the stronghold and called it the city of David. And David built the city all around from the Millo inward (2Sa. 5.9 ESV).

And David lived in the stronghold; therefore it was called the city of David. And he built the city all around from the Millo in complete circuit, and Joab repaired the rest of the city (1Chron. 11.7-8 ESV).

The “Millo” seemed to be terraces built on fill material that supported structures on top of it. Evidently, this southern flank needed a retaining structure over a precipice. Chandler Collins explores some of its archaeology in this series.

https://approachingjerusalem.substack.com/p/stepped-stone-structure-1