I am so happy that our blog at last was joined by another specialist in Norse mythology. From now on I hope to have with who to discuss all aspects of Norse Mythology.
But, before we start, I want to tell you, that my prime interests are runes and not Norse Gods. I am not worshiping these Gods in anyway, I just search knowledge about runes and their meaning. But, the connection of runes' meaning and Norse Gods is so strong, that I cannot avoid studying them. As you know, in the Norse mythology we speak about 9 words. The highest one is Asgard, the world of Odin, who is "the God of the Gods". We won't discuss here all 9 worlds, but you gave me a brilliant idea about a new post. But it will have to wait for a while, because the planned list is too long at the moment.
I want to dissapoint you. It seams that you knowledge in Norse Mythology are not enough deep. Yes, Valkiries were taking hero warriors over rainbow bridge called Bifrost to Valhala, which is situated in Asgard. Bifrost is the name of this bridge. It was guarded by Heimdall, who was one of the Odin's sons. Heimdall was that God, that stood between the mortals and Gods. He is the God, who revealed to mortals the Secret of Runes. Apart from guard duty, HEIMDALL is the ever-watchful sentinel who waits with his horn to announce the end of the world. His horn is called Gjail or Gjallar. When RAGNAROK arrives, he'll blow the most amazing note and the last battle will commence.
As about giants, they are kept at bay by Thor's hummer till Ragnarok will come and all 9 worlds will be involved in it.
I hope I answered your question. If not, I can give you the names of some of my favorites books, where you can find the answer that you are looking.
Thank again for your very interesting comment and question.