Are these impersonals?

[phil]

Rēgulus has been attacked by a 120 foot long monster, which seemed impervious to injury by normal weapons. Cōnfugiendum fuit ad māchinās advectīsque ballistīs et catapultīs, velut arx quaedam mūnīta, dēicendus hostis fuit.

War machines had to be resorted to, and with trebuchets and catapults (having been) brought up, the enemy (monster) had to be destroyed, just like some well defended fortress (would be).

Have I got the gist of this correct? Are there two impersonals in this snippet? (excuse again the crunchy English, I'm more interested in grammar)

Also I'm not 100% sure about the quaedam there 'just like a certain fortress'. There is no other arx mentioned anywhere, so it's not being compared to any other particular one.

[ptolemyauletes]

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.

[thesaurus]

Most Latin verbs may be used impersonally in the passive voice, especially Neuter and Intransitive verbs, which otherwise have no passive; as pugnatur, favetur, curritur, venitur...