I'm in a garden with Farah. The last water fountain was at the dark alley. Before entering the garden, the Prince and Farah saw Vizier, and the Prince rushed and wanted to kill him. After the cutscene, and after activating a lever, Farah gets in a door.
Now, I want to know where am I suppose to go next?
Almost forgot, it was where i first encounter with invisible enemies.

You'll probably have figured it out by the time you read this, but here goes...

That wall switch actually open two doors - the one for Farah and your exit. So, after the cutscene, pull the wall lever again, and quickly drop back down onto the box. Wall run from there to your left, towards a small balcony in the corner - there's the other door.

Ok, i'll try it.
Should i use the slow-time magic?

No, you won't need to slow time here - save your sand for later.

Thx.

Now I'm stuck right in the first area of Upper City. Where should I go?
There's a box with a cross on it, and it can't be moved.