In porta urbis parvum praesidium manet, ne hostes nocte subito intrent.

Yes. Here in is being used for location: in/at the gate. That means it takes the ablative.

The form is really portā (ablative singular of porta), but many Latin texts are printed without macrons, so portā appears simply as porta.

So:

• in portā = in / at the gate
• not nominative porta, even though it looks the same without the macron

Urbis is genitive singular of urbs, urbis = city.

It depends on porta and means of the city:

• porta urbis = the gate of the city
• in smoother English, the city gate

This is a very common Latin pattern: noun + genitive noun.

In this sentence, it means at the gate.

Latin often uses in + ablative for position in a place, and with something like a gate, English often prefers at rather than in.

So although the Latin is literally something like in the gate, the natural sense is:

• at the city gate

If Latin wanted to emphasize motion toward the gate, it would more likely use ad portam.

Because praesidium is a neuter singular noun.

The adjective parvus, parva, parvum must agree with the noun it modifies in:

• gender
• number
• case

So:

• praesidium = neuter singular nominative
• therefore parvum = neuter singular nominative

That is why the phrase is parvum praesidium = a small garrison / guard / force.

Praesidium has a range of meanings, including:

• protection
• defense
• guard
• garrison
• detachment of troops

In this sentence, since it is something that stays at the gate, the best sense is probably:

• a small guard
• a small garrison
• a small detachment

So it is not just the abstract idea of protection here; it refers to the people or force providing that protection.

Because its subject is praesidium, which is singular.

Even though praesidium refers to a group of people, Latin treats it grammatically as a singular noun, much like English can say:

• The army remains
• The crowd stays

So:

• parvum praesidium manet = the small garrison remains/stays

Ne introduces a negative purpose clause.

That means the clause tells you why the garrison remains at the gate:

• manet, ne ... intrent = it remains, so that ... may not enter
• or more literally/formally, it remains, lest ... enter

This is a very common Latin construction:

• ut
  • subjunctive = positive purpose
• ne
  • subjunctive = negative purpose

Because it is in a purpose clause introduced by ne.

Latin uses the subjunctive in purpose clauses:

• ne hostes ... intrent = so that the enemies may not enter

Also, the main verb is manet, a present tense verb, so Latin normally uses the present subjunctive in the dependent purpose clause:

• manet → primary sequence
• intrent → present subjunctive

Normally, for a negative purpose clause, Latin prefers ne, not ut non.

So:

• ne intrent = so that they may not enter / lest they enter

By contrast, ut non is more likely in other constructions, especially where non simply negates something inside the clause rather than marking standard negative purpose.

For a learner, the safest rule is:

• purpose clause
  • positive: ut
  • negative: ne

Hostes is nominative plural here.

It is the subject of intrent:

• hostes intrent = the enemies enter / may enter

A beginner might wonder whether it is accusative plural, since hostes can also be accusative plural in form. But here the verb intrent does not have a direct object; instead, hostes is clearly the one doing the action.

Because nocte is an ablative of time when.

Latin often uses the ablative by itself to express when something happens:

• nocte = at night
• die = by day / in the daytime
• tertia hora = at the third hour

So nocte does not need a preposition.

Here subito is an adverb, meaning suddenly.

It modifies intrent:

• subito intrent = enter suddenly

This is a useful word to recognize, because it can look a little like a noun or adjective form at first glance, but in sentences like this it functions adverbially.

Latin word order is much more flexible than English word order, because the endings show the grammatical relationships.

The sentence is arranged roughly like this:

• In portā urbis = location first
• parvum praesidium = subject next
• manet = main verb
• ne hostes nocte subito intrent = purpose clause

So the structure is:

• At the city gate, a small garrison remains, so that the enemies may not enter suddenly at night.

A few important points:

• Latin often puts important setting information early, so in portā urbis comes first.
• The main idea is completed by manet.
• Then the purpose clause explains why the garrison remains there.

Grammatically, yes: ne in a purpose clause is often explained as lest.

So a very literal translation would be:

• A small garrison remains at the city gate, lest the enemies enter suddenly at night.

But in natural English, people often prefer:

• so that the enemies do not enter suddenly at night
• to prevent the enemies from entering suddenly at night

So lest is a good grammar label, but not always the most natural everyday translation.