Poem question: Mezzo Cammin by Henry Wadsworth Longfellow?

It's conventional for poets to refer to their work as song. The "tower of song" here is the body of work the poet hoped to create. He hoped his tower would be tall and grand -- that is, he hoped that by this time in his life he would have written a large quantity of great poetry -- and he feels that what he has actually achieved is less than what he'd hoped for.