In this video we go over weapons, monsters and powerups which are all added via the Entity layer menu.
For a more thorough listing, see the Entity part of the Reference section.
Make sure to click "Compute Y for selected entities" at least for each newly added entity, or when the underlying sector has changed height. Otherwise the entities might end up floating high up in the air or be buried inside level geometry.
Enjoy this part since it allows you to populate your levels and create new layers of enjoyment and torture for everyone.