Diablo Wiki

Ancient Tower is a multi-level building rarely found on the Greyhollow Island in Act V of Diablo III, in Adventure Mode only.

It was once the gathering place of the island's cult, where sacrifices to Nereza were held. Now, it stands derelict.

The location is part of the island itself (called Ancient Tower Grounds), a smooth transition zone. One can enter through the main gate and proceed to interior, which consists of three parts:

Forsaken Repose is an alternate version of the Battlements, accessible only through a purple portal from Testament of My Failure tomes.

One can find High Cleric's Journal tomes on all the levels of the tower.