What is the main conflict in The Girl Who Owned a City by O.T. Nelson?

In The Girl Who Owned a City, a virus has killed off everyone over twelve. Ten-year-old Lisa Nelson takes on a leadership role, finding stores of food and organizing Grand Avenue for protection against gangs. After the local gang burns down her house, she gets the idea of making a defensible castle, and Lisa leads the way in turning the local high school into her own castle, a new city that she fights to run and keep.