Instead of stairs I will make ramps, you can use CSGBoxes to make stairs if you wish.
By adding a container for
Level components as a Node3D
Rename it to LevelContents
Put the contents inside it
As a child of LevelContents
It's currently a cube but we want a pyramid
Expand the Polygon property
Set the size property to 3
Now Scale and Rotate the Ramp
Move it to the next location
Drag and drop the Player scene
You can now explore your level
Add a Node3D as a child of MainLevel
Drag into it some Enemy scenes
Place the enemies around the map
Add a Node3D as a child of MainLevel
Drag into it some Pickup scenes
Place the pickups around the map
Add a NavigationRegion3D as a child of MainLevel
Make LevelContents Node a child of NavigationRegion
Select the NavigationRegion3D node
In the Navigation Mesh property
Select the NavigationRegion3D Node
In the Viewport select Bake NavigationMesh
Now Enemies can navigate the level
Grab the new enemy model alongside
Delete the two MeshInstances