HBP - Hogwarts Secrets

[Tonks]

No: HPFGUIDX 147711

Here is a thought provoking statement by DD that I found after 
rereading the section on LV's Request. (Page 431, US edition) DD 
says that he thinks LV wanted to teach at Hogwarts because: 
"The castle is a stronghold of ancient magic. Undoubtedly LV had 
penetrated many more of its secrets that most of the students who 
pass thought the palace, but he may have felt that there were still 
mysteries to unravel, stores of magic to tap."

Maybe we need to ponder what other secrets the castle 
holds. "Stronghold of ancient magic", bring to mind the "ancient 
magic" that protected Harry at Godrick's Hollow. Also maybe there is 
magic there that may have only been unlocked by DD. 

This leads me to another thought. Since DD was the only one that LV 
ever feared and DD's presents at Hogwarts kept everyone safe
  What 
does his death gain LV?  I had thought that it was just to get DD 
out of the way so that LV could get to Harry. But maybe it is not 
just about Harry, or not about Harry at all. 

Help me out folks. What do we know about Hogwarts?

1. It was the hiding place for the Stone that gave Eternal Life.
2. It was the safest place for the stone, even safer that Gringotts.
3. The Chamber of Secrets is there.
4. Trelawney is there, but I don't think she is that important even 
though DD was protecting her by keeping her there.

What else??  What is in Hogwarts that LV wants? Something or some 
knowledge that he needs?

Any ideas?

Tonks_op