Here is my theory about the ravens.

The Tower of London is built on White Hill or Tower Hill, name depending on era. As we know, ravens have lived there since time immemorial. Now, a curious bit of Welsh myth from the Mabiongion, in "Branwen uerch Llyr:

King Bran the Blessed died while at war with Ireland. (He was ancestor to King Arthur, BTW.) Now, being a god,* and more than that, a Celtic god, he possessed a magical head that stayed alive after the rest of him was dead. His brother Manawyddan, nephew Pryderi, and friend Taliesin, all returned home from the war with his head, feasting on magical isles and never growing older. Until the day that someone broke a taboo, that is. So, the head now dead, the three, plus four companions, take the head to the White Hill of London (now Tower Hill) and bury it as a protection against the successful invasion of Britain by outsiders. King Arthur, according to the Welsh Triads, dug it up out of arrogance, believing he could protect Britain on his own. Of course, Mordred kills him and the Saxons take over the island.

I mention this, because of what the name Bran means in Welsh--raven.

When I was in England, I asked a beefeater whether he knew this story. He didn't, but said he'd look into it out of curiosity.

Only one prisoner ever walked out of the Tower alive--Elizabeth I.

*To the Celts--as with the Norse--their gods can die. It's strange sort of mortal divine, but as the Celts simply saw death and life as equal passages, and one lead as easily to the other and vice versa, this isn't too surprising.