选which还是where的判断,就是看从句中缺什么成分
从这个句子里看,从句中缺少的是一个地点状语,这个从句相当于we enjoyed a splendid view of the lake there.
所以只能选where而不是which,假如用Which要加上介词,变成at which=where
I walked up to the top of the hill with my friends,where (at which) we enjoyed a splendid view of the lake.