[2020] how to build a cellar?

You can rotate some props. There's a rotate option in the utilities menu. Things like beds/stairs can be rotated. Ladders don't have a rotated version because they're really hard to read from the side. just looks like a pole. However, you can still build a cellar. Rotation doesn't affect whether the entity can climb the ladder or not. Just dig out your pit, put a roof or floor in the layer above, leaving one open and build a ladder under that open one. Then you've got a little cellar/basement!