Hostibus oppugnantibus, legio non recedit, sed turres et vallum defendit.

It is an ablative absolute.

That means it is a small phrase, grammatically a bit separate from the main clause, that gives the circumstances of the action. Here it means something like:

• with the enemies attacking
• while the enemies are attacking
• when the enemies attack

In this sentence:

• hostibus = by/with the enemies in the ablative plural
• oppugnantibus = attacking, also in the ablative plural

So the phrase sets the scene for the main action: while the enemies are attacking, the legion does not retreat...

Because the phrase is an ablative absolute, and both words in an ablative absolute normally go into the ablative case.

So:

• hostes = nominative or accusative plural, the enemies
• hostibus = ablative plural, with/by/from the enemies

Since Latin is using this special construction, hostibus is required.

Because oppugnantibus is a present active participle, not a finite verb.

Compare:

• oppugnant = they attack
  This is a normal verb.
• oppugnantes / oppugnantibus = attacking
  This is a participle, a verbal adjective.

In an ablative absolute, Latin usually uses:

• a noun/pronoun in the ablative
• plus a participle agreeing with it

So hostibus oppugnantibus literally means with the enemies attacking.

They match in:

• case: ablative
• number: plural
• gender: masculine here, because it refers to hostes

This agreement is exactly what you expect from an adjective or participle in Latin. Since a participle behaves like an adjective, it must agree with the noun it describes.

Oppugnantibus is a present participle.

A present participle usually shows action happening at the same time as the main verb. So here the sense is:

• while the enemies are attacking
• the legion does not retreat
• but defends the towers and rampart

So the attacking and the defending are happening simultaneously.

Because legio is the subject of both verbs, and it is singular.

• legio = the legion
• recedit = it retreats / does retreat
• defendit = it defends

Latin verbs must agree with their subject in number and person, so singular legio takes singular verbs.

Latin usually puts non directly before the word it negates, especially before a verb. So:

• non recedit = does not retreat

Latin word order is flexible, and you may sometimes see non in other positions for emphasis, but non recedit is the normal and straightforward order.

Sed means but.

It contrasts the two actions:

• legio non recedit = the legion does not retreat
• sed turres et vallum defendit = but it defends the towers and the rampart

So sed marks a strong contrast: the legion is not withdrawing; instead, it is holding its defenses.

Because they are two different nouns with different declensions and numbers:

• turres = towers, accusative plural
• vallum = rampart/palisade, accusative singular

Both are direct objects of defendit.

So the verb governs both:

• defendit turres
• defendit vallum

Latin often combines them as: turres et vallum defendit = defends the towers and the rampart

Because it is the direct object of defendit.

The noun is turris, turris (feminine, third declension), and its accusative plural is turres.

So:

• nominative plural: turres = the towers as subject
• accusative plural: turres = the towers as object

In this noun, the nominative plural and accusative plural happen to look the same.

Not necessarily in the way an English speaker might first imagine.

Vallum often refers to a rampart, entrenchment, or fortification line as a single defensive structure. So even though English might sometimes say the walls or the defenses, Latin can use singular vallum for the defensive earthwork or rampart as one unit.

So vallum singular is perfectly natural.

Yes. Latin often puts background or setting information first.

By beginning with hostibus oppugnantibus, the sentence first gives the military situation:

• with the enemies attacking

Then it states the main response:

• the legion does not retreat
• but defends the towers and rampart

This order is very natural in Latin and helps frame the scene before giving the main action.

Yes. An ablative absolute often allows several natural English translations, depending on context. For example:

• while the enemies are attacking
• when the enemies attack
• with the enemies attacking
• as the enemies attack

All of these reflect the same basic Latin structure. The important thing is that it gives the circumstances under which the main clause happens.

They are:

• hostibus → hostis, hostis = enemy
• oppugnantibus → from oppugno, oppugnare, oppugnavi, oppugnatus = attack, assault
• legio → legio, legionis = legion
• recedit → from recedo, recedere, recessi, recessus = retreat, go back
• turres → turris, turris = tower
• vallum → vallum, valli = rampart, palisade, defensive wall
• defendit → from defendo, defendere, defendi, defensus = defend

Knowing the dictionary forms helps you see why the case endings look the way they do.

It breaks down like this:

1. Hostibus oppugnantibus
   = ablative absolute, giving the situation

2. legio non recedit
   = first main clause

3. sed turres et vallum defendit
   = second main clause, contrasted with the first by sed

So the overall pattern is:

[background circumstance], [main statement], but [contrasting main statement].

That is a very common and useful Latin sentence pattern.