This 1929 Modern Mechanics HOWTO explains to young people how they can dig their own pirate's cave, complete with working fireplace. The author explains that mothers can allow their sons (yes, it was all pretty gendered back then in 1929-land) to build such a clubhouse without fear, because the design will prevent cave-ins. Articles like this fill me with sorrow and delight: sorrow because you couldn't include such a place in a work of fiction (letalone a factual article aimed at children) today; delight at the adventure that those who followed these directions back in the 20s and 30s must have had.
Save all the flat stones for the fireplace, unless bricks are available. The latter will make a better fireplace, however, without mortar. The roof or ceiling joists should extend at least a foot on each side of the excavation. The ridge support is made up of two two-by-fours laid one on top of the other, as shown in the diagram. The roof boards should be covered with tar paper or old canvas, or in a pinch, several layers of newspapers. At one end of the roof, tack heavy wire screen under the gable, and further protect this with a row of slats set at an angle. These are to partially support large stones placed against them to conceal the vent. If the stones are big enough they will not impede air circulation to any great extent. A trench is dug for the stove-pipe and, when this is laid, covered over again with dirt. Of course, it will be an advantage to have the chimney as far away from the cave as your supply of stove-pipe will permit. However, be sure that the top of the chimney is one or two feet higher than the stove. Otherwise your draft will be sluggish. Stones should be piled around the chimney to hide it, and it wouldn't be a bad idea to throw over the chimney itself some old junk, such as rusty washboilers, etc., that will not interfere with draft. In case a potential enemy sees smoke rising he naturally would assume it to be a rubbish fire.