I'm struggling to find the title of this book I read a few times growing up and really enjoyed. Main character, pretty sure is named Ghendt, is in the Stone age, hunting, and comes across a tower. Inside he meets some sort of divine force that starts training him. I think it takes the form of a cat outside the tower, and he refers to her as "Em" since she has emerald eyes, and when he's outside the tower she can communicate telepathically with him. He then travels through doors in the tower to different time periods like the Bronze and Iron age. He has to find certain people and affect events in certain ways, while there is a dark force with its own avatars working against him.
Tried all sorts of ways to Google that but coming up blank. If anyone has any ideas, I'd be happy to hear, would like to read the book again, maybe get the kid interested in reading as well.