Once a Castle — Sarah Arthur

Worlds collide and lives are changed as portals between our reality and the magical land of Ternival fray in the spellbinding sequel to the Christy Award finalist Once a Queen, called “a fresh, delightful new tale for our wonder-hungry era” by Mitali Perkins.

A few years after Eva Joyce and Frankie Addison discovered the existence of portals to other worlds, Frankie's younger siblings, together with some new friends, make discoveries that will change their lives, and at least two worlds, forever.

Jack, Tilly, and Elspeth Addison just want to stay out of trouble (well, mostly, at least), but trouble keeps finding them. 

Meanwhile, Arash, a newcomer to the Addisons' English village, tries to discover the secret his grandfather is hiding while keeping his own secrets from everyone. And Zahra, a Ternivali teenager, faces a crisis in her world, but before she can put things right, she's rudely catapulted into the weirdest place she's ever seen -- where everyone seems to think she's the weird one.

Soon their stories get tangled together, and the teens must discover if they can bridge their differences and find, in themselves and one another, what it takes to put things right. Because it's not only their lives at stake but also the fate of their home -- and of more than one kingdom.