I am currently doing a comprehension on the destruction of Numantia (Florus I.34). The comprehension question I am stuck on is as follows: 'State and explain the case of the word fossa'. The sentence it is in is:
cum Scipio fossa atque lorica quattuorque castris circumdatos fames premeret, Numantini a duce oraverunt proelium, ut eos tamquam viros occideret; ubi non impetrabant, eruptionem facere constituerunt.
I assumed fossa is in the ablative case to agree with lorica and castris, and links with circumdatos to have some kind of meaning like "surrounded by a ditch...", but have no idea how to explain this. Is there a grammatical term (e.g. ablative of comparison, ablative absolute) to explain this use? If I'm completely wrong and it's not the ablative after all, or I've gotten the usage wrong, what is it then?