What happens in chapter 7 of The Age of the Ninth?

The Eagle of the Ninth (Book One of the Eagle of the Ninth Trilogy, by Rosemary Suttcliff) is a historical novel set in Roman Britain during the early Republic:

Sometime about A.D. 117, the Ninth Legion, which was stationed at Eburacum, where York now stands, marched north to deal with a rising among the Caledonian tribes, and was never heard of again.

The plot centers around the Centurion Marcus Flavius Aquila. We learn that his father had soldiered with the Ninth Legion in Judea, Egypt, and Britain for ten years, rising to the rank of Cohort Commander, and was with the Ninth (also known as the Ninth Hispana) when they marched north beyond what is now Hadrian's Wall.
When he was eighteen, Marcus joined the Roman Legion. After rising to his father's rank and being posted to the frontier in Britain, his leg is severely wounded when he and the men in his garrison defend their fort from a local uprising. Discharged from his military duties, Marcus goes to live with his Uncle Aquila to allow his leg to mend. When his leg heals—and when he is no longer content to simply exist day after uneventful day—Marcus sets out with his slave, Esca (a former gladiator who he ultimately frees), to discover the truth about the Ninth's and his father's disappearance.
At the beginning of chapter 7, "Two Worlds Meeting," Marcus is living in his Uncle Aquila's home near Calleva, an important crossroads city west of Londinium (present-day London).
Marcus desperately needs something to do after being cooped up indoors all winter, so he sets about repairing his uncle's souvenir Celtic weapons—the only decorations his uncle allows on the walls of his home.
Occupied with replacing the leather straps on a light cavalry buckler (a small, round shield), Marcus eventually notices a young British girl standing among the fruit trees looking at him. The girl looks cold to Marcus, so he gives her his old military cloak, which she quickly wraps around herself. Marcus learns that her name is Cottia:

"My aunt and uncle call me Camilla, but my real name is Cottia," said the girl. "They like everything to be Roman, you see."

Conversing first in Latin, then switching to English, Marcus and Cottia get to know each other. Marcus learned that Cottia hated living with her Aunt Valeria:

Only I hate living with my aunt; I hate living in a town full of straight lines, and being shut up inside brick walls, and being called Camilla; and I hate - hate - hate it when they try to make me pretend to be a Roman maiden and forget my own tribe and my own father!

They talk about Cottia's parents, her Iceni heritage, and how Cottia lies awake at night thinking about her family farm:

And I lie on my bed and think - and think - about my home, and the marsh birds flighting down from the north in the Fall of the Leaf, and the brood mares with their foals in my father's runs. I remember all the things I'm not supposed to remember"

She moves closer to Marcus and says that she likes his cloak; "I like being inside your cloak . . . It feels warm and safe, as a bird must feel inside its own feathers." However, they are interrupted by the shrill cry of Nissa, Cottia's nurse.
Cottia returns the cloak to Marcus, but begs him to let her return. Marcus answers:

"Come when it pleases you - and I shall be glad of your coming," Marcus said quickly.
"I will come tomorrow," Cottia told him, and turned to the old rampart slople, carrying herself like a queen.

Esca returns to the villa after his errands, and he and Marcus discuss what Roman rule brought to the conquered people:

"But these things that Rome had to give, are they not good thing?" Marcus demanded.
"These be all good things," Esca agreed. "But the price is too high"
"The price? Freedom?"
"Yes - and other things."

Esca shows Marcus a Roman-made dagger sheath embossed with a repeating pattern of "little round stiff flower[s]," made by someone, he says:

who had lived so long under the wings of Rome - he and his fathers before him - that he had forgotten the ways and the spirit of his own people.

Esca compares the dagger sheath to the Celtic buckler that Marcus laid aside when Cottia appeared to him. The man who carved the buckler, says Esca, knew things that the Roman people did not (or had known, but forgot). Esca follows this by saying:

You cannot expect the man who made this shield to live easily under the rule of the man who worked the sheath of this dagger.

Marcus thinks to himself that the distance between the world of the the man who fashioned the dagger sheath and the world of the man who carved the shield was considerable. However, he considers that:

between individual people, individual people like Esca and Marcus and Cottia, the distance narrowed so that it ceased to matter.