。
She stood still among the castle ruins.
Literal
She [topic-は] castle [possessive-の] ruins [possessive-の] inside [in-に] was-standing-still.
たたずむ (佇む) means 'to stand still, to linger quietly' — it implies a contemplative or reflective posture, more poetic than simply 立つ ('to stand'). 廃墟 (はいきょ, 'ruins') evokes a sense of history and decay. The ~ていた sets a vivid narrative scene.