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.