The Way Up is Death by Dan Hanks

With no understanding of what the tower is or why they’ve been chosen, thirteen seemingly random strangers must battle to reach the top of a tower before the countdown clock reaches zero… and the world faces a potentially catastrophic fate.

When a mysterious tower appears in the skies over England, thirteen strangers are pulled from their lives to stand before it as a countdown begins. Above the doorway is one word: ASCEND.

As they try to understand why they've been chosen and what the tower is, it soon becomes clear the only way out of this for everyone is... up. 

And so begins a race to the top with the group fighting to hold on to its humanity, through sinking ships, haunted houses and other waking nightmares. Can they each overcome their differences and learn to work together or does the winner take all? What does the tower want of them and what is the price to escape?