After watching first Shrek, I got interested in one detail:
Can you build a castle which is surrounded by lava, using medieval technology?
For scope of this question: Imagine ultimate wealthy king/queen (resources are not a problem). This kingdom wants to build castle to treasure its wealth. What if we build the castle guarded by lava?
The kingdom has access to anything which could be plausibly made before year 1500 CE. Also assume there is suitable volcano.
Can the kingdom build it?
Lava is a "hot fluid" [citation needed]. The technology to handle the "liquid" part is obviously available during medieval era, as irrigation is already known for centuries. For example, the first dutch polder was build from 1609 to 1612 with windmills. Digging a hole somewhere so that it can thereafter be filled with liquid is really not a challenge. Organizing the liquid flow is not a complex issue either.
That said, the "hot" part is more tricky. The area where the lava flows must resist the high temperatures, and no technology is available to protect people once the lava is in place. So this may lead to some adaptations :
On the overall, this is achievable if you have good engineers and a stable flow of liquid lava. However the resulting place will be hot and hard to enter, even if friend. Depending of the land size, it could end up in a tropical paradise (if you do it around a mountain, for example), or a balrog™'s lair...
In practice, the area close of the lava may look like hell (which can obviously be cool!), with red-glowing borders reaching high-temperature from the surrounding lava. Expect air to be filled with some toxic and stinking smokes due to sulfur, and nearby waters to become acid, not drinkable, but eventually colorful like the Kelitumu lakes. Most animals will leave, and nearly all plants die. Your visitors may be reluctant to come by...
One last thing : should your lava supply vary, or its fluidity not be top-quality, the lava will solidify in your structure and it will be another nightmarish hell to repair it and make it work again.
I think the hardest problem would be that lava doesn't exactly sit nicely like water does. Its constantly depositing rock as the lower edges cool. Your moat would constantly be changing shapes as lava from the volcano flows through your channels.
I got the privilege of photographing the lava flows in Hawaii. One of the things they warned me before we started was that lava is unpredictable. It's very easy to get caught up in the lava in your camera's eye, trying to get the lighting perfect, and fail to realize that another flow of lava behind you has just redirected itself towards your feet.
Another issue would be the Hydrogen Sulphide gas (H2S) that is common around volcanic events.
For one thing, H2S is an explosive gas. No lights in the castle after dark. No cooking fires, although you've got plenty of lava to cook your food for you.
Another thing is that H2S smells bad. It's often referred to as 'rotten egg gas'. You can start to smell it at less than 1 part per million (ppm).
That's only a minor problem though. It is also a toxic gas. The 'Short Term Exposure Limit' for H2S is 15 ppm.
The smell would cease to bother you after about 100 ppm, as it basically kills your olfactory nerves, permanently ruining your sense of smell.
At about 300 ppm, you are pretty much on your way to death, if not dead already.
H2S is heavier than air, so it would settle in the lowest parts of the castle. Dungeons and areas enclosed by the wall, rooms on lower floors etc.
Nasty stuff. I don't think I'd want to live in that castle.
It's impossible with medieval technology. Even today I would assume it as impossible without a great energy source or great thermal isolation of the "riverbed". Otherwise, the lava will lose energy, solidify at its edges and bottom and eventually overflow its "riverbed" leading to catastrophe.
In fact, I would expect such a lava river to eventually create a lava tube.
