So, there is a fireplace at coordinates X20 Y19 1 and a switch at X21 Y19 1. The fire is not burning by default, but if you pull the switch, it lights up, you can also extinguish it with the swith. Does anybody know what is the point of this fire?

Well, the area with the fireplace is the armory after all. It's just a decoration or a scenery.

However, if you attack fire with your weapons, you can increase your skills. Beware of attacking with your bare hands - you'll take some minor damage!

Yes, this serves no purpose except boosting your skill with a weapon.

How do you use ruin make it work???"?"