SOLVED. kids discover that they're brother and sister, trapped in a dungeon on a school trip, merlin involved? [s]

[Lana]

so this book is definitely for children, and I feel like it had something to do with merlin. this girl and boy visit a dungeon or a castle on a trip and get sucked into this story about a brother and sister (a prince and princess?) who had been jailed but mysteriously disappeared. and then they discover that it's actually them in a past life, and merlin is involved. I thought it might have been a RL Stine book, but maybe not. does this sound familiar to anyone?

[Angharad]

Sounds like RL Stine's A Night in Terror Tower.

[Lana]

YES, you're awesome. thaaaaanks.