A hang-glider or a gyro-glider would suffice also, with the purpose of actually working.

I was hoping it wouldn't require a motor, because I doubt a motor could be built in medieval times.

If it could be built, how much manpower and resources would go into one single one? Would it be viable to mass produce in military applications (surprise attack on lower altitude enemies)? How reliable would the thing(s) work?

Gyroglider

I would say no. It will be impossible to get bearings for the rotor that can turn easy enough and avoid running hot or seizing up. Plus the problems with the glider.

Fixed-Wing Glider

It will be problematic to get the structure strong enough and light enough at the same time. Getting materials for the wings will difficult, too. Imported bamboo and silk?

The next problem are the control surfaces. That could be overcome with a basic hang glider, but for anything larger than that you probably need steel wires (and bearings, again).

A hang-glider could certainly be built. Back in the 1970s, people made their own out of bamboo and canvas. Otto Lilienthal did too.

And they all died with alarming regularity too. It turns out that designing the right stall characteristics is not so straightforward, and if you get it wrong then you simply fall out of the sky and die. Even with 1970s knowledge of aeronautics, it still took a decade to get reasonably safe hang-gliders. So building the hang-glider, yes. Knowing what dimensions to build a working hang-glider, probably not.

As for engines, absolutely not. Nothing before the internal combustion engine had the power-to-weight ratio to let an aircraft sustain flight. So they aren't going to be flying around and dropping bombs.

If we assume they managed to build a working hang-glider though, there is another use for it, which is reconnaissance. With a carriage fitted with a winch and several fast horses it would be possible to get a hang-glider to a reasonable altitude from which you could see the enemy's army. A Montgolfier balloon would be a much easier solution though - still an anachronism, but less so, and much easier to launch too.

Since it's been determined that a motorised machine is out of the question, and also that even until recently, gliders were pretty much death traps, we might as well have fun answering the subquestion involving getting a flying weapon from a higher to a lower place with some stealth.

To which end,

allow me to introduce the Trebuchet Launched Warbird!

In this image from an ancient manuscript in the Puddlian Library, we can see the Warbird awaiting launch. The graceful and powerful descent of the trebuchet weight lifts the long arm thus yanking the Warbird by the short tow rope. The massive weight drives the light craft to an incredible upward velocity.

Up up and over the edge of high Fortress Rockberg! Her graceful arc will send the bird out over the jagged rocks and her silent downward glide will surely take those Foreign buggers down below by surprise!

There are, of course, a few ... ahem ... minor kinks to be worked out. What with the trip being one-way and all. But surely the lads of the Rockbergian Army shall be queuing up to do their bit for King and Country all the same!

In a medieval world with current knowledge of aerodynamics, yes. Nature provides enough tough materials to build a structure that a single human could use to glide a bit.

Problems:

1) With current knowledge, a design could, within a few months and with small loss of life and limb, be brought to perfection, but with then contemporary knowledge, it would be very hard, and could only be bought by many years of experimenting, and a host of lost pilots. You'd need to get medieval to find more testpilots. Hey...

2) This is about a glider. Not a good one, either. Optimistically a glide ratio of 3:1? (Every 3 meters you go forward are bought with one meter height loss (https://www.britannica.com/sports/hang-gliding)). High ground was coveted even then, So having to find an accessible yet steep hill overlooking your target, undefended, and then schlepping your gear up there, to surprise the enemy below? Not that many chances for that.

3) Using mechanisms to gain height, like winches, counterweights or mooring in a stiff breeze would add many more years of testing, invalidating many of the pure glider designs because they now also be able to survive these forces.

4) Birds, stones, arrows, and many other flying things were known in medieval times, so a commander getting credible info about the enemy being able to glide 'like a chicken with clipped wings' (or some such) could identify possible ambush-places that were made viable by such technology. And you have just been chucking people from your castle walls tied to tents for years now. This will not be a secret. People will be somewhat prepared.

5) Whether by time travelling aeronautics engineer (cum carpentry enthusiast (with a pinch of not getting burned as a witch)), or pure aristocratic bull-headedness, you now have gliders that can be packed like tents, a mobile launch system, fighter-pilots, and a bird-related nickname. Whence now?

I have a question: What counts as a "motor" for the purpose of this topic? A world record for human powered flight was awarded 30 years ago to a plane called Daedalus, which was flown by a super-fit Greek cyclist. I realize this isn't the same as a gyroplane, but it would have been possible to build during medieval time, assuming you had a King's budget.

Before you posted the illustration, I was picturing a small glider of some kind being deployed by hot-air balloon. While I could find no evidence of thermodynamic knowledge in medieval Europe, paper hot-air balloons existed in China pre- Marco Polo. (I believe they were used in funeral ceremony.)

Depending on how much of a stretch you're willing to accept, a combination of these ideas would theoretically make it "possible."
I hope my answer is acceptable. I've never responded to a question on Stack Exchange before.