This is an older strategy for falling beneath stormwind, however it STILL works!
Sorry if this is a repost, however I searched and did not find a similar thread, and sorry if the pictures are too big
Here we go:
First we start out by the fountain just outside stormwind bank.
Then we head over to the 2 torches on the wall
Now we run straight into the wall next to the left torch.
Now as you're facing the wall strafe right (default is the 'E' key) and jump just before the ground seems to slope down.
Now you should land on an invisible platform on the torch like so:
Then jump on top of the torch.
From there turn to your left toward the flat concrete surface.
Then run onto the surface...
And you fall:
There we have it...You're under stormwind city
Thanks for looking!!