Are these sentences correct:
1-This building was constructed in the 15th century, an audacious undertaking at the time.
2-He was attacked by three men, beaten up and left for dead, a cowardly crime.
3-This long poem was written in only a few days at most, a superhuman feat.
You could move 'at most' back a bit and enclose it in commas. You could also use different punctuation instead of the final commas.