Are both 回了 and 回来了 correct responses to 你回天津了吗 (have you returned to Tianjin)?
回 = back; 来 = come; 去 = go
回来 = come back; 回去 = go back
回天津去了 = "have gone back to Tianjin"
回天津来了 = "have came back to Tianjin"



  你回天津了吗？


If the question is asked in 天津, that would mean it is actually asking "你回天津(来)了吗？ (have you came back to Tianjin?), then you can answer "回来了" (yes, I have came back) or simply "回了" [yes, I have (came back)]

If the question is asked anywhere else, that would mean it is actually asking "你回天津(去)了吗？ (have you gone back to Tianjin?) then you can answer "回去了" (yes, I have gone back) or simply "回了" [yes, I have (gone  back)]

In conclusion: 

Depend on either you are in Tianjin or not,  "回了" could means "I had came back"(if you are in Tianjin)  or "I had gone back".(if you are not in Tianjin)