There was this book I read years ago (2000-2009, possibly even older) about a boy who finds some sort of portal door.
It takes him to this world where a tribe is fighting to survive by digging for gold and getting food from the people that enslaved them. If one of them doesn't get enough gold they face the consequences and die.
Then another world in the book is like a colosseum type story and he is forced to fight a lion to survive.
Another world was more medieval I believe.
The cover had a brick wall with a blue looking portal blob and the main character right next to it!