Tower of the Dead - The Dead Series, #1

Five years after the still unexplained event that altered the world, killing millions, the residents of what is now known as Tower Island, go about their lives with an uneasy, but confident feeling of safety. The zombies, or 'crits' as they have become known, are no longer a danger to the community, thanks to the efforts of Captain Evan Stewart and his troops. But something strange about the dead's recent behavior has him on edge.
Though the dead are massing near the gate, they make no effort to gain access to the island. Attending a party in the elite Tower, a twenty-six story apartment complex, Evan notices muzzle flashes near the perimeter gate on the causeway leading to the island. Fearing an attack, he attempts to contact his troops, but finds the breach has reached the island, dead are roaming the streets. Unable to reach the Island Defense Forces, Evan tries to get to the ground but discovers the Tower is already overrun.
Unarmed, with thousands of somehow organized dead between him and escape, Evan must find a way to lead the Tower residents to safety. As the muzzle flashes of his troops diminish, he realizes the battle is lost-no help will be coming. With the Tower overrun, Evan decides there is only one way down. They must descend the Tower from the outside.