What Greek formation could block the narrow Thermopylae pass with ease?

The Greek formation that could block the narrow Thermopylae pass with ease was the hoplite phalanx.

A hoplite phalanx arranged heavily armed infantry in close ranks. Each soldier carried a large round shield, a spear, and protective armor; the overlapping shields and projecting spear points created a formidable barrier in a confined space.

Thermopylae suited this formation because cliffs and the Malian Gulf restricted movement and largely neutralized the Persian advantage in numbers and cavalry. The Greeks could rotate units and meet frontal attacks without exposing an open flank.

The phalanx was not invincible. Its major weakness was the Anopaea mountain path, which allowed Persian troops to outflank the position. Once that route was revealed, the Greek defense at the pass could no longer hold. A shield wall is a broad descriptive term, while testudo is specifically associated with Roman formations.