Prep Phrase at the beginning of the sentence...

In the background, there is a magnificent gray tower with a clock, ticking slowly, at its peak.

Is that punctuation correct?  I'm not sure about the rules for prep phrases that begin a sentence.

A prepositional phrase at the beginning of a sentence should not be followed by comma unless it is extremely long, in my opinion.  I would not do it in this case.  It's only three words.

CJ