Per its title, Psalm 57 was written when David fled from Saul into what?

Answer: A cave

Psalm 57 is titled "A Miktam of David, when he fled from Saul into the cave."

“For the choirmaster. To the tune of “Do Not Destroy.” A Miktam of David, when he fled from Saul into the cave. Have mercy on me, O God, have mercy, for in You my soul takes refuge. In the shadow of Your wings I will take shelter until the danger has passed.”— Psalm 57:1 (BSB)