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 gliders. So building the glider, yes. Knowing what dimensions to build a working 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 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 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!