I read this scifi novel/short story around the 2000s in a public library, and I recall the following plot points which could be helpful in its identification:

The space traders (not sure if traders, but certainly a group of people with spacefaring abilities) somehow ended up on a medieval human planet, where the main civilization was a decaying empire much like the medieval Byzantines.

The decaying empire often fought skirmishes with the barbarians, who would handily defeat the heavily armed and armoured empire knights by drawing out their battle lines, retreating, and picking them off when they were tired and overextended.

The space traders managed to revitalise the empire by training a spearman army from the remaining peasants. This army then defeated the barbarians by holding the line against repeated assaults during a siege on the capital.

The story ends with the space traders being "exiled" to the medieval planet for violating a Prime Directive equivalent.

I believe this was a relatively old book when I had read it in the library around that time. The author was IIRC pretty famous in the science fiction genre as well, but I cannot remember any details on that. I'm rather certain about the plot points above, though.

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    Some of this sounds a bit like Janissaries. – DavidW Oct 19 '19 at 16:14
  • @DavidW, Janissaries had sequels, the first volume did not end that way. – o.m. Oct 19 '19 at 16:26
  • @o.m. It's been a long time; I'm most certainly willing to admit I could be confused. I was thinking of the first place the soldiers landed - Tamerthon or something like that? - who were primarily archers and who needed tactics and training to stand up to the knights. I know they used pikemen against the Romans in a later book, but I thought there was a Crecy-like battle against the knights in the first. – DavidW Oct 19 '19 at 16:47
  • @DavidW, I'd have to re-read all involved books to be sure. – o.m. Oct 20 '19 at 4:37
  • @DavidW, King David's Spaceship is clearly a match for the question. I didn't re-read Janissaries. – o.m. Oct 27 '19 at 18:34

Jerry Pournelle, King David's Spaceship.

This story is distantly connected to the more famous Mote in God's Eye by Niven and Pournelle. In a different edge of the Empire, a planet is trying to avoid getting taken over by Imperial traders. Their two-prong approach involves a spaceship on their own, and traveling on Imperial trade ships to an even less developed planet where the military action takes place.

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    The mission to the second planet is to visit an old library from an earlier interstellar empire. By the rules of the new empire, you can only import or sell things to a planet at or below its own technology level. Since the barbarian planet is lower on the scale than King David's planet, he can import anything at all from the barbarian world - the rules don't account for left overs from the previous empire. King David's people were sent to get information from the old library to help with the primary goal. – JRE Oct 19 '19 at 16:49
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    The primary goal is to build a spaceship, since planets with space travel capability are taken into the new empire on better terms than planets without space travel. – JRE Oct 19 '19 at 16:50
  • @JRE, I hard tried to minimize spoilers. Well, people who look here are probably beyond that ... – o.m. Oct 19 '19 at 17:33
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    "King David's Spaceship" is over 40 years old. I think we're past spoilers here. – JRE Oct 19 '19 at 17:38

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