"When the Renaissance brought back the concept of static symmetry, the great axial plans opened on a mountain, the sea, a fine colonnade or building, or groups of buildings, or else when long and extended the avenue itself became the view, the trees closing in always beyond, until at the end of long travel a turn or a closure of permanency brought it to an end."

1. "when long and extended"

What became "long and extended"? --- The avenue???

2. I cannot understand the blue sentence at all. PLEASE HELP!!!

Read about perspective here:
Perspective (visual)

For example, the parallel lines of a railway track are perceived by a
standing human being as meeting at a distant point at the horizon.

Thank you so much, Marius Hancu. It's quite simple, but I didn't understand it. :-)