Dark Souls Remastered | Location of Watchtower Basement Key & Divine Ember

Neither of these items are mandatory to beat the game, but both can be very useful. They are looted at the same time in the tower right after the Moonlight Butterfly boss fight.

Divine Ember:

The Divine Ember allows the modifying (ascension) of a standard weapon to a divine weapon. A divine weapon prevents skeletons from respawning after killed.

Watchtower Basement Key

This unlocks the door at the bottom of the tower, where Havel the Rock is located. This door provides a shortcut to Undead Parish.