I like it overall. There are few plot points I question: why is HP not utilizing his siblings' talents, why didn't James and Lily search for their lost children (who cares if the aurors were looking...they should have been seeking them out on their own)?, and whey would HP order his siblings away when they could choose to go underground in ravenclaw's chamber until the danger had passed?
Sorry for the late reply; RL got in the way. The reason Shyamal isn't using his siblings talents is purely due to the fact that he doesn't want to get involved. In fact, if it hadn't been for the risk of injury, he probably wouldn't have told Damian about him being Anthanasius. James and Lily did search (James was an Auror at the time) for Evie but they were never able to find her body. They did have bodies for Daniel and Jonathon which is why they were presumed dead. No one has the full story yet regarding what happened that night in August 1990. You will have to read between the lines and put the tiny hints together.
Considering that fact that they discovered the Castellum in the middle of the semester, suddenly disappearing would raise flags within the Ministry would could lead to potential tensions between the Elf Kingdom, the Romanian Ministry, and the British Ministry. Another factor would be that they're 15 and younger. They are still teenagers and are prone to mistakes, especially when they are the ones in the middle of it. As they say, hindsight is 20-20.
Thanks for the review!