The source of this sentence is not a poem but a song.
"Half the city's snow fell across the river and sea, and I was leaping on my horse outside the wall" is not a complete poem, but a line of lyrics, from the lyrics of the song "Shenyuan Outside": "Half the city is in smoke Soldiers of sand are approaching at the bottom of the pond, fighting for the world with gold and iron horses. Tens of thousands of bones will be withered. How many white hair will be gone? Half of the city's snow will fall through the river and the sea, but I am leaping on my horse outside the wall."
This lyrics describes an ancient war scene, full of tragic and desolate atmosphere. Among them, "Half the city's snow fell across the river and sea, but I jumped on my horse outside the wall" expresses the protagonist's heroic ambition to bravely kill the enemy in the war and fight bloody battles on the battlefield.