Question: Fort Lyon was constructed south of a city whose population was what in the census of 2010?
Sentence (from "Fort Lyon (Virginia)"): From its position on one of the highest points south of Alexandria, the fort overlooked Telegraph Road, the Columbia Turnpike, the Orange and Alexandria Railroad, the Little River Turnpike, and the southern approaches to the city of Alexandria, the largest settlement in Union-occupied Northern Virginia.
Relevant evidence: No