Is this sentence grammatically correct?
This is a correct sentence! The reason is that you have two complete independent clauses (basically two sentence) that are combined with a conjunction, but. Together, they make a compound sentence. For the first sentence, the main idea is that they played.
Well in this clause is an adverb to describe the manner in which they played. For the second half of the sentence, the ideas are written in passive voice.
It would be better to say who defeated them and rearrange the words like this: “but their opponents defeated them” to help the reader better understand your meaning. However, the way that you have written it here is just fine!
