I would like to know what "the gigantic gates and lines of soldiers that protected the castle that was the Soviet embassy" means in the following sentences:
We whizzed off, speeding seamlessly and effortlessly along Ujazdowskie Avenue. We passed the run-down palaces of the long-forgotten aristocracy, the Łazienki Gardens with my hidden deer, and the gigantic gates and lines of soldiers that protected the castle that was the Soviet embassy. After that the city turned sparse. We passed endless stretches of identical blocks, blokowisko upon blokowisko with mud fields in-between, where riotous hordes of children played. We passed factories, smoking behemoths, big and solemn like sooty churches. The radio was on, playing something by the Velvet Underground. Nico sang in her low, litanic voice, bells ringing and a guitar jittering, like a flickering mirage.
In this novel which is set in the 1980's in Poland under the socialist regime, where homosexuality was socially unacceptable, the protagonist Ludwik (a university graduate) left Poland in 1981 to live in the United States of America. And he remembers what it was like back then in Poland, where he decided to spend some days at the country house of Hania's family with his lover Janusz, Hania's brother Maksio, and Maksio's girlfriend Agata. So they (the five of them) drove towards the country house in the car.
In this part, I wonder it would be right to understand that what protected the castle that had once been the Soviet embassy were the gigantic gates and the lines of soldiers, or that only the lines of soldiers protected the castle.
I am an English learner from South Korea, so thank you for your patience in advance as I may not know obvious things. I would very much appreciate your help. :)