Caminhámos junto à muralha antiga até chegar a uma lagoa tranquila.

In European Portuguese:

• caminhamos = present tense – we walk / we are walking
• caminhámos = past tense (pretérito perfeito simples) – we walked

The accent in caminhámos:

• marks the stress on -há- (ca-mi-NHÁ-mos),
• and distinguishes the past from the present in writing.

So in this sentence, caminhámos clearly means the action is finished in the past: we walked.

Portuguese has two common past tenses:

• pretérito perfeito (here: caminhámos) – a completed action in the past, seen as a whole event:
  → We walked (and finished walking).

• pretérito imperfeito (here would be caminhávamos) – ongoing, repeated or background action in the past:
  → We were walking / we used to walk.

In this sentence, the walk is seen as one completed route from the wall to the lagoon, so caminhámos (pretérito perfeito) is the natural choice.

Yes, you can say Nós caminhámos…, but in Portuguese it’s normal to drop subject pronouns when the verb form already shows who the subject is.

• caminhámos can only mean we walked, so nós is not needed.
• Adding Nós is possible if you want to emphasise the subject, e.g. to contrast with others:
  Nós caminhámos, eles foram de carro. – We walked, they went by car.

Here, junto à muralha antiga means by / next to / close to the old wall.

Rough nuances:

• junto a – right by, very close, almost touching; slightly more formal/literary.
• perto de – near, but not necessarily right beside.
• ao lado de – beside, at the side of (more specifically “at the side of” rather than just “near”).

So:

• Caminhámos junto à muralha antiga suggests walking right next to the wall, along it.
• Caminhámos perto da muralha antiga only says you were somewhere near the wall.

à is a contraction:

• a (preposition “to / at / by”)
• a (feminine singular definite article “the”)
  = à

So:

• junto à muralha = junto a + a muralha = by the wall

If you wrote junto a muralha, it would sound wrong to a native speaker, because muralha is a countable noun and normally needs the article (a muralha) here.

Both are used in European Portuguese and both mean by / close to the wall:

• junto a is a bit more formal / written.
• junto de is very common in speech.

So you can say:

• Caminhámos junto à muralha antiga.
• Caminhámos junto da muralha antiga.

In this context, the meaning is practically the same.

There are two different things:

1. junto a – a fixed expression (preposition) meaning by / close to. Here junto does not agree with the subject.

   • Caminhámos junto à muralha. – We walked by the wall.

2. juntos – an adjective/adverb meaning together, which agrees with the subject (nós → juntos).

   • Caminhámos juntos. – We walked together.

You could even combine them:

• Caminhámos juntos, junto à muralha antiga. – We walked together, by the old wall.

In Portuguese, the default position of adjectives is after the noun:

• muralha antiga – old wall
• lagoa tranquila – calm/quiet lagoon

They can go before the noun, but then you often add a stylistic or emotional nuance:

• a antiga muralha – can sound more like the former wall (emphasis on its age in a historical sense)
• a tranquila lagoa – more poetic / literary, emphasising tranquil as a characteristic.

In this sentence, the post‑noun position (muralha antiga, lagoa tranquila) is the most neutral and natural.

Because adjectives in Portuguese agree in gender and number with the noun:

• muralha is feminine singular → muralha antiga
• lagoa is feminine singular → lagoa tranquila

Both nouns end in -a and are grammatically feminine, so the adjectives also take the -a ending: antiga, tranquila.

The subject is still “we” (nós), carried over from caminhámos.

Portuguese often uses preposition + infinitive to mean “until (someone) does X”:

• Caminhámos… até chegar a uma lagoa tranquila.
  → We walked… until (we) reached a calm lagoon.

You could also use the personal infinitive to show the subject more explicitly:

• …até chegarmos a uma lagoa tranquila.

Both are grammatically fine in European Portuguese:

• até chegar – subject understood from context (here: “we”).
• até chegarmos – subject “we” is marked on the verb; slightly more explicit.

Because até here is used as a preposition (“up to / until”), and prepositions in Portuguese are followed by an infinitive, not a finite (conjugated) verb:

• até + infinitive → até chegar – until reaching / until (we) reached

If you conjugate the verb (e.g. chegámos), you would normally need a conjunction até que:

• Caminhámos junto à muralha antiga até que chegámos a uma lagoa tranquila.

This is also correct, but até + infinitive is more compact and very common in writing.

In European Portuguese, the normal pattern is:

• chegar a + place – to arrive at / to reach (a place)
  → chegar a Lisboa, chegar a casa, chegar a uma lagoa

In Brazilian Portuguese, many speakers say chegar em in everyday speech, but in Portugal, chegar em for places sounds wrong or at least very odd.

So in Portugal you should stick to chegar a when you talk about arriving somewhere.