I think the quaedam might well be rendered as 'some kind of fortress.'
Just one impersonal in the sentence - confugiendum fuit = it had to be fled
This is a gerundive being used with the verb to be, which indicates a certain necessity... sometimes called the passive periphrastic.
There is a second passive periphrastic in deicendus hostis fuit, but this is not impersonal... the subject here is hostis
A good way to spot whether one of these passive periphrastics is being used impersonally is the ending.
um is likely to be impersonal, since it will probably be using 'it' as its subject. (unless there is a neuter subject in the sentence)
us or a, or i or ae, are not going to impersonal, since they wiill have some masculine or feminine noun, pronoun, adjective as their subject.
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