Depending on which leg bone is broken, in the 13th century (and much of history) this would be a moderate to deadly problem. Up until more modern medicine, there were limitations in surgery and ever looming threat of untreatable infection. I'll start right out that most 13th century fractures are not going to heal properly by modern standards; leading to crooked or lumpy limbs, unless it was non displacement and minor.

A tl;dr is that a minor displacement fracture of the tibia or fibia could be slightly manipulated back into place and immobilized then hope no further complication arises. However, a femur fracture posed serious issues.

I'll start with the immediate, most serious concern - the femur. Historically, most femur fractures were a serious issue with high mortality, studies reporting 60% to 80% mortality rates. Up until World War 1's introduction of the traction splint, a femur break was difficult to adequately treat due to human anatomy. It's the largest bone surrounded by the largest muscles, with the large femoral artery and its blood supply. If the fracture type created bone fragments/blades, this is going to cause severe bleeding and if the femoral artery is damaged in most of history; its a ticket to six feet under.

The fall may cause an open or compound fracture. This is the most critical that could happen. A Comminuted fracture is also critical. A 13th century surgeon is going to have extreme difficulty treating this for the femur. Even if they somehow managed to set it, the bleeding, swelling, infection, and potential compartment syndrome will likely be mortal. Speaking of setting, advanced techniques like pins and plates were unknown. It was a matter of lining it back up and hoping for the best. Amputating at the hip was also highly risky because of the blood loss and following infection possibility. They had no antibiotics and their methods to stop blood loss were comparably barbaric to modern times. This is why even until 1914, up to 80% of serious femur fractures would be lethal.

A distal fracture, hairline fracture, minor transverse fractures are going to be possible to treat; or to be more precise...keep stable and hope for the best. If it is not a displacement fracture then most of the risk is swelling, blood loss (into the calf/quad), and inner infection. Again a minor tibia or fibia fracture was a lot less risky than femur fracture. If the wound was in the lower leg and became infected and there was no signs of it stopping, they would simply attempt amputation in hope that it doesn't continue the infection.

Regarding the immediate swelling, there's not much a 13th century surgeon could do, except in extreme cases to cut in to relieve pressure. Which would again open the risk for infection. If compartment syndrome set in resulting in infection, the only real chance at survival was amputation.

Regarding infection, germ theory was no yet developed, but they knew around wounds that the blood could go bad, which they would (ineffectively) treat with leeching or bloodletting. They were not aware of sterilizing any utensils, but were aware of some Roman/Greek remedies, that would involve herbs like sage or even poultices from urine, wine or boiled wine, and even honey. They knew to use potable water to clean the wound, replace bandages and also believed that bad air led to fouling; so depending on the practitioner - they may be correctly or incorrectly recommending leaving a wound exposed. As a result of bad air causing issues, they would attempt to treat the air itself with incense and oils - to drive away toxicity and in more superstitious folks, evil spirits wishing to do harm. Furthermore, a physician would have studied the classics and known of the 'four humors' and would be aware that pus and foul odor wounds would need to be removed/cut away. One other distinction I have yet to make was the most scholarly and advanced persons would actually be averse to performing the medical procedures themselves. The actual treatment would be applied by a barber-surgeon or chirurgeon which was seen as a bit below in prestige than a physician themselves.

Revisiting setting the bone, it was not an exact science at all. No x-rays meant they could not actually see what was going on, and would do their best to painfully manipulate it back into place, and then use a wooden splint and plaster to immobilize. Again, if it were a compound or a more severe displacement fracture this success is limited. They may not even be able to actually set it and just hold it in place, wherever it was. Although tibia and fibia fractures had a better chance at a union than a femur, it still would not necessarily heal straight. Most leg fractures left the patient with life-long limps or gait issues.

A further addendum is that the closest most castles/small towns had to a doctor would have been a barber-surgeon, a priest, and/or a medicine man/woman (basically..a midwife that could be accused of witchcraft).

Regarding pain relief is actually a topic that I come across as one of the most contentious. Some medieval authors put forward certain herbs as analgesics, but modern science seems to mostly shy away from this. Also a debated subject was the availability of poppy derivatives. Other areas of the world knew of it, but its not 100% certain until Paracelsus' discoveries in the 16th century.

https://history.amedd.army.mil/booksdocs/vietnam/OrthoVietnam/CH02.html

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4523509/

https://oll.libertyfund.org/titles/hippocrates-the-writings-of-hippocrates-and-galen (classical medical writers had a serious influence on medieval medicine).