。
She turned left at the corner.
Literal
She [topic-は] corner [at-で] left [target-に] turned.
角で marks the corner as the location where the turning happens — で is the standard particle for the location of an event or action. 左に曲がる ('turn left,' literally 'turn into the left') uses に for the direction of the turn: the destination of the rotational change. This に-の-direction pattern parallels に for results-of-change (大人になる 'become an adult,' プロになる 'turn pro') — the new orientation is the result. Note that 角 ('corner') is the same kanji used in 三角 ('triangle') and 角度 ('angle') — the math-sense of 'angle' shares its character with the everyday street-corner sense, and even the related verb 曲がる ('bend, turn') feels visually consistent.