Salve rustymason,
Is this also good?:
A: Videntes magna moena urbis ea non oppugnavimus.
The case of “videntes magna moena” is nominative or accusative, isn’t it? But it must be ablative! Therefore: videntibus magnis moenibus!
I also disagree with Interaxus on the “function” of the ablativus absolutus. At least, I learned it this way: the Ablativus Absolutus is a participial construction. What distinguishes it from the regular Participium Coniunctum is that it isn’t as closely connected to the “main activity” of the sentence. For example:
- urbem a hostis oppugnatam egressus sumus.
(“After the city was attacked by the enemies, we left it.” Or: “Because the city was attacked …” etc.)
urbem, beeing an accusative object, is part of the main activity. => Participium Coniunctum.
- Legatus a Caesare praemissus a Britannis in vincula coniectus est. (“The legatus, who was sent by Caesar, was put into prison by the Brits.”)
As you see, Legatus is closely connected to main activity, since it is the subject of the sentence.
Because of this “connectedness” to the main activity, it is called Participium Coniunctum.
Now there are parts of a sentence, that are detached from the main activity: you could leave them out, without changing the meaning of the sentence too much. Appearantly the Romans were very fond of participial constructions, so they had to have one for this case aswell (or something like that): the Ablativus Absolutus (from absolvere: to detach).
- oppido a hostis deleto Marcus et Claudia Romam confugerunt.
(“After the city had been destroyed by the enemies, Marcus and Claudia fled to Rome.”)
The main activitiy here is: Marcus and Claudia fled (to Rome). The participle oppido deleto clearly isn’t as connected to the main activity as in the previous sentences.
Now the meaning of either P.C. or Abl. Abs. isn’t only a temporal one. You can translate it using many kinds of subordinate clauses (causal, final etc.), as a relative clause (in the case of the P.C.) or as a plain participle. All depending on the context.
The use of the present participle denotes simultaneity to the main activity, while the use of the past participle denotes anteriority.
Magnis moenibus videntibus would mean something “the moment (while) we saw the great walls, we [did this and that] …”
I think: in your sentence you’re intuitivly trying to connect the participle with the main activity: by connecting moena with urbs and through the use of eam. I think, because that is the more english way to think.
I hope, I could help. Please excuse any errors: it was late in the night!
regards