Nocte eadem porta a militibus custoditur.

Not necessarily. Latin often uses the ablative of time when with no preposition to express when something happens.

So nocte means at night / on that night / during the night, depending on context.

This is very common with words for time, for example:

• eo die = on that day
• hac nocte = on this night / tonight
• prima luce = at first light

So Nocte eadem is a normal Latin way to say on that same night.

Because eadem has to agree with nocte.

The word idem, eadem, idem means the same, and like an adjective, it must match the noun it goes with in:

• gender
• number
• case

Here:

• nox, noctis is feminine
• nocte is singular ablative
• so the matching form is eadem = feminine singular ablative

So nocte eadem means on that same night.

Porta is nominative singular.

We know this because it is the subject of the verb custoditur. In a passive sentence, the thing receiving the action becomes the subject.

So here:

• porta = the gate → subject
• custoditur = is guarded

That is why it is porta, not portam.

Because the sentence is in the passive voice.

In English:

• The soldiers guard the gate = active
• The gate is guarded by the soldiers = passive

Latin works the same way.

So in the active version, you would have:

• milites portam custodiunt = the soldiers guard the gate

But in the passive version:

• porta a militibus custoditur = the gate is guarded by the soldiers

The gate is not doing the action, but it is still the grammatical subject because the verb is passive.

Because with a passive verb, the person or people doing the action are usually expressed with a/ab + ablative. This is called the ablative of personal agent.

So:

• a militibus = by the soldiers

If Latin used milites, that would be nominative, which would suggest the soldiers were the subject. But the subject here is porta.

A useful contrast:

• milites portam custodiunt = the soldiers guard the gate
• porta a militibus custoditur = the gate is guarded by the soldiers

Yes, -ibus can be either dative plural or ablative plural in many third-declension nouns.

But here it must be ablative plural, because it follows a, and a/ab takes the ablative.

So:

• a militibus = by the soldiers

not something dative like to/for the soldiers.

Both are forms of the same preposition.

A common rule is:

• a before a consonant
• ab before a vowel or h

Since militibus begins with m, a militibus is the expected form.

You may sometimes see variation in actual Latin, but for a learner, this is the basic rule to remember.

Custoditur is:

• third person singular
• present tense
• passive voice
• indicative mood

It comes from custodire, meaning to guard.

So custoditur means:

• is guarded
• or is being guarded

Because it is singular, it matches the singular subject porta.

Yes. Latin word order is much freer than English word order because the endings show the grammatical relationships.

For example, these would mean essentially the same thing:

• Nocte eadem porta a militibus custoditur
• Eadem nocte porta a militibus custoditur
• Porta nocte eadem a militibus custoditur

Latin often puts the verb near the end, as here, but that is a style tendency rather than a strict rule.

The order in your sentence slightly emphasizes the time phrase first: On that same night...

Many learners first meet the phrase as eadem nocte, because adjective + noun or noun + adjective are both possible, and eadem nocte is a very natural order.

But nocte eadem is also perfectly good Latin. Latin often moves words around for emphasis, rhythm, or style.

So the important thing is not the order, but the agreement:

• nocte = ablative singular feminine
• eadem = ablative singular feminine

They belong together no matter which comes first.

Yes, sometimes. Latin authors often use the historical present, where a present tense verb is used in past narrative to make events feel more vivid.

So in isolation, custoditur normally means is guarded. But in a narrative context, it could be translated as something like was guarded if that fits the passage.

Still, grammatically, the form itself is present passive.