。
She used to live at 56 Russell Square.
Literal
She [topic-は] Russell Square 56 number [at-に] living-was.
A straightforward past habitual sentence. ラッセルスクエア is the transliterated London address. ５６番地 uses 番地 (ばんち, 'house number') — the standard Japanese address component. 住んでいた is the past progressive of 住む, indicating former residence. The sentence demonstrates how Japanese handles foreign addresses.