I downvoted for the most of the same reasons above. I think it was kinda weird from the beggining. I mean, firstly it is all pretty convienient, you know? She threw the backpack at the boulder, then the boulder shook and then she grabbed her hammer (?) and just destroyed the boulder with it. It wasn't a pile of rocks, it wasn't a cracked wall, it was an entire boulder and she broke it with only a hammer? Ok, I guess, but I really couldn't picture that scenario as it was described to me.
My second problem was the description of the buildings inside the place. They all felt the same, just cracked walls, broken furniture and all that. If they all had an important role in the narrative, ok, but the only one I felt that had that role was the castle. These descriptions got pretty boring after a while, as they were so detailed for no reason. I already knew the place was destroyed, you know?
About the ending, I don't understand how they were flooded at all. It was a cave, like a massive cave, and it had only one entrance? And still, they closed that entrance (which the girl easily opened but the whole civilization there couldn't) and then what? Imagine a plastic bottle, then close the lid. How are you going to fill that bottle with water?
Anyway, the writing was great overall, still. I believe these problems can be easily solved, so good luck in the future!!