Please explain the quote “always peeping slyly down at Scrooge.”

The full sentence in the quote appears as follows:

The ancient tower of a church, whose gruff old bell was always peeping slily down at Scrooge out of a gothic window in the wall, became invisible, and struck the hours and quarters in the clouds, with tremulous vibrations afterwards as if its teeth were chattering in its frozen head up there.

It is Christmas Eve, and Scrooge is in his home. Dickens is describing how cold and foggy it is outside. It is so foggy that the horses pulling the carriages can't see. It is also so foggy that the church tower can't be seen.
Dickens is personifying the church tower when he says it is always peeping slyly down on Scrooge. To personify an object is to describe it as if it is human. Towers don't peep or spy on people: people do that. Towers certainly aren't sly: that is a human characteristic.
Dickens calls the tower "gruff" and compares the ringing of its bells to teeth chattering in its head as if it is human, further personifying it.
However, because it is a church tower, the words have an extra meaning. It seems that the spirt of God keeps glancing or "peeping" down on Scrooge, awaiting a moment when he might be softened enough to have his heart reopened to kindness and compassion.