"Danville" is the 2nd Dimension counterpart of the 1st Dimension's Danville. It first appeared in Phineas and Ferb The Movie: Across the 2nd Dimension.
At some point prior to the movie, the Doofenshmirtz family became the dictators of the city, with the help of Heinz Doofenshmirtz's armies of Norm Bots and Charlene Doofenshmirtz's army of Animal Borgs. During their reign, a resistance movement against the family's dictatorial rule was formed by Candace Flynn, but their efforts in defeating them didn't prove that successful.
This paid off as Heinz was arrested, and peace returning to the city shortly, even when after Heinz escaped from prison with the help from Charlene and their daughter Vanessa two months later.