Ans : The Astronomy Tower. 
I'm mostly lonely except for midnight.
As the sun crosses horizon and twilight ascends,
I slowly become more active.
Astronomy tower is usually deserted and out of bounds for students during the day time.Students visit the tower mostly for Astronomy classes around midnight when stars are best seen.
I've witnessed many things over the years.
Strength of a friendship,courage and loyalty.
and of promises that were not forgotten.
This part talks about the incidents that happened there.Strength of friendship,courage --This refers to Harry and Hermione's visit to send Norbert off to save Hagrid from trouble.
Loyalty,and Promises not forgotten-- This refers to the incidents happened during the battle of Astronomy Tower.About how Snape was always loyal to Dumbledore and kept his word despite the dangerous position he put himself in.Also in reference to the promise Snape made to Draco's Mom that he'd look after him.
Once two friends paid me a visit,
To get their friend out of trouble.
But Alas! sad for them,it was not their day
Though they did what was to be done,
They got caught and brought shame to their house.
Now rack your brains and spell my name.!
Again Harry's and Hermione's visit in detail.
Edited by QuietlyLoud - 11 years ago