Yes.  Thank you.

Your sentence effectively sets up an intriguing premise, but it could be refined for clarity and impact. Here’s a possible revision:
"Martin arrives in Calridge with his new girlfriend—but she’s no ordinary girl; she’s a Giant. Soon, she brings the entire city to its knees."
Comments:

Grammar & Spelling: "Girlfreind" should be corrected to "girlfriend," and "it's" should be "its" (possessive, not a contraction of "it is").
Flow & Readability: "Shows up" is a bit casual; "arrives" feels more polished. The semicolon instead of a comma before "she’s a Giant" creates a stronger impact.
Emphasis & Suspense: "Soon" helps build anticipation for the city's downfall.

Would you like it to sound even more dramatic or poetic?