Per its title, Psalm 56 was written when the Philistines seized David in which city?

Answer: Gath

Psalm 56 is "a Miktam of David, when the Philistines seized him in Gath."

“For the choirmaster. To the tune of “A Dove on Distant Oaks.” A Miktam of David, when the Philistines seized him in Gath. Be merciful to me, O God, for men are hounding me; all day they press their attack.”— Psalm 56:1 (BSB)