Why is the verb singular rather than plural

Question

Part of a sentence in the above exercise says "Donde se encerraba a los enemigos del castillo" Should encerraba not be plural i.e. encerraban? Is it singular because " a los enemigos" is indirect? I'm confused. Why not fueron encerrados?

Accepted Answer

Hola Kevin
This is a very interesting question. This sentence is a "se impersonal" sentence (not following the construction of a reflexive se sentence). We keep the verb in the singular form even when the object is plural, in this case: "los enemigos". 
We do this when the object is "personal" (as opposed to a thing) and requires the preposition "a". 
Here is an article that talks about this from the R.A.E that explains this grammatical point. (The answer is on the last two bullet points)
I hope it helps.
Saludos cordiales
Inma