The Greeks did defend that path, Herodotus 7.217.2-3 points out that a detachment of a thousand Phocians had volunteered to defend the path. The Greeks were clearly aware of its potential as a vulnerability to their position but the Persian force under Hydarnes drove them from their position. The news of this event, which apparently made the Greek defence untenable, precipitated the withdrawal of most Greek forces aside from Leonidas and his royal guard, who as we well know were killed in the final stand.