I have never thought of this line in reference to the Parthenon, and to me that makes no sense. Here's my interpretation:
The way I've always viewed this line is as saying "every high point in hardships that I must face." The poem goes on to say:
Like a sick eagle looking at the sky.
The speaker is ready to die, obviously, because of these godlike hardships. He is like an eagle in that he's tired of flying, sick of having to keep going, since it's such an effort.
The hard times that the speaker is going through are being described as pinnacles, like walking over the crest of a hill. It gets harder and harder ("steeper") because the tension releases at the pinnacle, but he cannot rest because he sees the next pinnacle, the next godlike hardship, coming up.
It's like the light at the end of the tunnel turning out to be another tunnel full of darkness.