La catedral estaba llena de gente, así que entramos primero en una iglesia pequeña del barrio.

This is a very common Spanish tense pattern:

• estaba = imperfect
• entramos = preterite

Here, estaba llena de gente gives the background situation: the cathedral was full of people. It describes a state, not a completed action.

Then entramos gives the main event that happened next: we went in / entered. That is a completed action, so the preterite is used.

So the sentence has this structure:

• background or situation: La catedral estaba llena de gente
• event that happened because of that situation: así que entramos...

This imperfect + preterite contrast is extremely common in Spanish storytelling.

Because lleno/a normally goes with de in Spanish.

So the standard pattern is:

• lleno de agua
• lleno de coches
• lleno de turistas
• lleno de gente

So:

• La catedral estaba llena de gente = The cathedral was full of people

Using con here would sound unnatural in standard Spanish.

A good rule to remember is:

• lleno/a de + noun = full of + noun

Both are possible, but gente is more natural and common in this kind of sentence.

• gente = people, crowds, people in general
• personas = persons/people, a bit more neutral or countable

So:

• llena de gente sounds very natural
• llena de personas is grammatical, but less idiomatic here

In everyday Spanish, when talking about a place being crowded, gente is the usual choice.

Grammatically, gente is singular, even though it refers to many people.

That means if gente is the subject, verbs and adjectives usually agree in the singular:

• La gente está cansada
• Mucha gente vino

In your sentence, gente is not the subject, but it is still a singular noun after de:

• llena de gente

This is one of those words that learners often need to get used to: it means many people, but grammar treats it as singular.

Así que means so, therefore, or so then.

It introduces a consequence:

• La catedral estaba llena de gente, así que... = The cathedral was full of people, so...

In this sentence, it connects cause and result:

• cause: the cathedral was full
• result: they went first into a small church in the neighborhood

It is very common in spoken and written Spanish.

Compare:

• así que = so
• por eso = for that reason / that’s why
• entonces = then / so

All can be similar, but así que fits very naturally here.

Good question. With places, both patterns exist, but in Spain entrar en is very common and often preferred in careful standard Spanish.

So these are both heard:

• Entramos en la iglesia
• Entramos la iglesia

But in Spain, entrar en + place sounds especially natural and standard.

So in your sentence:

• entramos primero en una iglesia pequeña del barrio

is completely normal Spanish from Spain.

A useful learner rule is:

• with a place, entrar en is always safe

Here primero means first or first of all.

So:

• entramos primero en una iglesia pequeña del barrio = we first went into a small church in the neighborhood

It suggests this was the first place they entered, probably before going somewhere else later.

Its position is flexible. You could also see:

• Primero entramos en una iglesia pequeña del barrio
• Entramos en una iglesia pequeña del barrio primero

But the version in your sentence sounds very natural.

Placing primero before the prepositional phrase en una iglesia... makes it clear that it modifies the action entramos.

Del barrio is the contraction of:

• de + el = del

Literally, una iglesia pequeña del barrio means a small church of the neighborhood.

In natural English, that usually comes out as:

• a small church in the neighborhood
• a small local church

The idea is that it is one of the churches belonging to or located in that neighborhood.

So del barrio gives a local, familiar sense: not just any church, but one from that area.

In Spanish, adjective position can affect style or nuance.

• iglesia pequeña = a small church, physically small
• pequeña iglesia can also mean a small church, but it may sound more literary, emphatic, or slightly subjective depending on context

In everyday descriptive Spanish, the adjective usually comes after the noun:

• casa grande
• calle estrecha
• iglesia pequeña

So una iglesia pequeña is the most neutral and natural way to say a small church here.

This is about definite vs indefinite articles.

• la catedral = the cathedral
• una iglesia pequeña = a small church

La catedral suggests a specific cathedral that is known or identifiable, probably the town’s main cathedral.

Una iglesia pequeña introduces a church that is not being identified specifically beforehand. It is just one small church in the neighborhood.

So the contrast makes sense:

• the well-known cathedral: la catedral
• one unspecified small church: una iglesia pequeña

This is the same kind of contrast you would often have in English:

• the cathedral
• a small church

You could, but it would change the feel.

• estaba llena de gente describes the situation as background: the cathedral was full
• estuvo llena de gente presents it more as a completed, bounded event: it was full of people for a certain period

In this sentence, the speaker is setting the scene and then telling what happened next, so estaba is the more natural choice.

That is why:

• La catedral estaba llena de gente, así que...

sounds better than using estuvo here.