is this the factory（ where ）he worked ten years ago?

第一句先行词指地点,关系副词在从句中作地点状语,所以用where.
第二句先行词虽然指地点,关系代词在从句中作visit的宾语,用that.
不一定先行词指地点、时间或reason时,就用关系副词when、where、why了,一定要看关系代词在从句中作的成分,如果关系代词在从句中作的是主语或宾语,仍然用which或that.比较下列各组句子：
I like to take a vacation in the mountains,which is quiet and beautiful.
I like to take a vacation in the mountains,where there are a lot of plants.
Never will I forget the day which we spent together.
Never will I forget the day when we worked together.
That is the reason that my son gave me for not going to college.
That is the reason why my son decided not to go to college.
希望我能帮助你解疑释惑.