How high a human can jump in lower gravity is rather complex.
Assuming, as a first-order approximation, that the upward speed when leaving the ground is the same as in normal gravity, you can jump three times as high. But what does that mean? Good high jumpers on Earth can clear a 2 m bar, but they do that more or less lying down, with their centres of mass very low over the bar. Their centres of mass when leaving the ground is roughly 1 m above the ground, so the centre of gravity is only lifted about 1 m. This is what we will have to triple when gravity is one-third normal. A high jumper on this planet will thus be able to clear a 4 meter bar.
However, there' more to it: The assumption of having the same jump speed as on Earth is too simplistic. A high jumper fights gravity when flexing muscles to achieve jump speed, so the force provided by the legs is a combination of the force needed to counteract gravity and the force that provides the upward speed. On Earth, the force needed to counteract gravity is 10 m/s/s, and the acceleration to reach the speed required to lift the centre of 1 m is another 20 m/s/s, assuming that the acceleration of the body centre of mass is done over half a meter (crouching to standing), for a total of 30 m/s/s. If only 3.33 m/s/s is needed to counteract gravity, there will be 26.67 m/s/s left to actually accelerate the body upwards, for a jump speed of 4 m/s. In one-third gravity, this is enough to lift the centre of mass 4 m, meaning that a good high jumper can clear a 5 m bar.
To clear a wall, you just need to be able to get your hands on the top of the wall and swing yourself over, which adds another meter to how tall the wall must be to prevent people from jump-and-swing over it. Walls hence must be at least 6 meters tall to prevent easy access by jumping.
This, of course, assumes that the jumper isn't wearing heavy gear, but a sword and light armor doesn't add all that much to the weight of a person and hence doesn't change the equation all that much, either.
Pole vaulting, however, would add more meters to how high you can jump. Since pole vaulting relies on turning the kinetic energy of your running speed into potential enery, the energy provided by pole will remain unchanged with lower gravity, assuming you can run just as fast as on Earth. That energy would however propel your body three times as high. Record pole vaulters can clear 6 meters, hence lifting the centre of mass 5 meters. By pole vaulting, a soldier on your planet could hence feasibly clear a 16 m wall; possibly a bit more, since the lower gravity might allow faster running. Coming down the other side might hurt, though as there is unlikely to be a thick, soft mat there, and it corresponds to falling roughly two stories on Earth. If you can land on the top of the wall, there will be no problem, though, so to be safe, defenders should probably build their walls at least 20 m high.