The castle looked like on another hill.

本句like 是连词，引导方式状语从句，由于用于looked 之后，也可看作表语从句。等于as if。

这个句子属于连词后省略了成分：it was （这个省略是有条件的。即：主语it 跟主句的主语相同，且是be 动词）

The castle looked like it was on another hill.

The castle looked as if it was/were on another hill.

方式从句as if 中，省略成分的例句：

She talked to the stranger as if (she were) absent-minded. 她和陌生人谈话时似乎心不在焉。

He opened the drawer，as if (he was) in search of something important. 他打开抽屉仿佛要找什么重要的东西。

She stood at the gate as if (she was) waiting for someone. 她站在门口好像在等人。

The teacher shouted at the boy as if (he was) angry. 老师朝孩子大喊大叫好似很生气。