Lily Quench and the Treasure of Mote Ely — Natalie Jane Prior

Kidnapped and taken back into the past to a crumbling castle in the middle of a creepy marsh, Lily Quench searches for the long-lost treasure of Mote Ely -- and a way back to her own time.
 
Locked in a dungeon, attacked by a dragon, and befriended by her eccentric great-great-great-great-grandmother, Lily finds that enemies can sometimes be friends -- and that old friends can unexpectedly turn into enemies.