A solid three and a half stars.
This starts off quickly, set in a richly detailed medieval-type city, on a planet still at this stage. Farmer takes the time to build a convincing environment, full of meticulous tidbits and interesting characters.
Alan Green, the titular character, is an Earthman, trapped on planet for years after the starship he was on malfunctioned and he was forced there by lifeboat. He has been enslaved as concubine to the local duchess of the court. Now he learns that two other earthmen have arrived in a distant city, where they have been captured as "demons". He must find away to escape his slavery, and reach the other earthmen before they are killed, which is far easier said than done.
All in all, a nice little planetary adventure. If you enjoy the work of Leigh Brackett, Edgar Rice Burroughs, or L. Sprague Decamp, or fun 50s SF, chances are you will like this.
The first part, which takes place in the two cities are excellent; each of them is nicely done. There is, however, a slight dropoff as he journeys across the plains to the other city. It's readable enough, just not as rich in elaborate details which make planetary adventures so appealing. The wonders discovered in this part only barely touched on, revealing just enough to move the plot along. This was slightlywewell. disappointing. The end is more than a bit rushed, but satisfying.
I will say the characterizations are above average for the period as well.
It's far from perfect--the prose varies from good to a bit cheesy at times, but for the most part, very readable for what it is. Recommended if you're looking for a good fun, read. Above average adventure SF.