We're going to want to make a couple of caulk brushes - rather than create them with a random texture then caulk them, we'll set the texture before creating the brushes: click on the Caulk texture in the textures window.

Make a brush as shown in this picture.

Press ctrl+tab to get a side view.   Press 4 to reduce the grid scale, and then reduce the height of your floor down to very flat:

Press ctrl+tab twice to get back to the normal 2D view.  Press ESC.

Draw another brush as shown.  This one will be of the right height already as you will immediately see in the 3D window :)

Press ESC.  Select the environment ceiling brush and Hide it - you will only accidentally keep selecting it otherwise.  You can reveal hidden brushes with shift+H.

In both of those cases you can usually create an efficient floor space, in the knowledge of what the player will actually be able to see.  We'll get to this later.

So for now we're being a little quick and dirty, but it will have a negligible effect on FPS so don't worry.

Let's move our walls up a notch so that they sit on the new floor.

Select all the walls in the 3D window by shift+clicking them all one after the other.

Press ctrl+tab to get a side view.

Move the walls up one notch so they rest on the floor brushes.

Press ESC, ctrl+tab twice, and right-click/drag the building back into view if it has wandered off centre.

Let's texture the floor.  In the 3D view, ctrl+shift+click a floor brush face.

Then ctrl+shift+alt+click the other brush's floor face.

Then click Textures/egypt/egypt_floor_sd and click the block-16sq texture.

Remember you can adjust the relative sizes of the Radiant windows so you can access more easily what you want at any given time.  So I made the textures window bigger, clicked the texture I wanted, and then made it back to smaller again.  Also I find the Texture Window Scale of 50% to usually be the most workable.

Press ESC to deselect the faces and press 8 to go back to a big grid scale.  Always worth doing as a habit.

You can also see that by texturing the lower edge of the longer floor brush, half of its length will be obscured by the square floor brush, which is something we can easily avoid and it demonstrates another option available to the mapper, the Clipper Tool:

Select the large floor brush, and either press X or click the button shown:

Click on the vertices shown in this picture, starting with the uppermost one.  When you click on the first one, the number 1 will appear next to it.  When you click on the second, the number 2 will appear, and the brush will be shown with a dividing line defined by where you clicked 1 and 2, with one chunk shown yellow and the other still red.

The Clipper Tool has two functions.  It can...

1. Crop an unwanted chunk of a brush: if you were to press Return (don't in this instance), the yellow section would remain and the red would be deleted.
2. Split a brush into two: if you press shift+Return (yes please, do it now) the brush is merely split along the line defined.

Both chunks remain selected and the cropper tool remains active.  Press ESC (or X or click the button again) to turn off the cropper.

Press ESC to deselect the brushes.

Now we can texture all the wall rim without having some texture wastefully drawn but not visible.

Apply a suitable texture, say Textures/town/town_wall town_c61a.  Press ESC.

Select the indicated wall in the 3D window, and press ctrl+tab to see it side on.

Press 5 for a smaller grid scale, then use the clipper tool and define a cut line as shown:

Press shift+return to divide the brush into 2.

In the 3D window, click on the smaller chunk to deselect it, leaving the other selected and the clipper active.  Then define another cut as shown.

Press shift+return to divide the brush into 2.

In the 3D window, click on the larger chunk to deselect it, leaving the other selected and the clipper active.  Then define another cut as shown.

This time we want to make an empty door space, but the yellow chunk is the wrong one to keep - so press ctrl+return to swap the yellow marker over, then press Return.

Press ESC twice now, as we are done clipping.

We have an opening.  The faces of the opening are caulk so we must do something about it.

For now, we won't bother putting in a door frame, we'll be a bit quick and dirty and texture the visible caulk.  This is slightly wasteful, because the two side faces extend up beside the small chunk of wall over the doorway.  Later when we make the door, we'll fix this.  As I said, sometimes it won't be worth the grief in trying to stop everything from being drawn if it won't be seen - but if you do this where practicable, you'll keep the FPS up and everyone playing will be grateful that your map doesn't play like running through custard :)

Select the 3 caulked faces that surround the door opening.  Apply Textures/wood wood_c01 and press ESC.

This leads us to another useful technique, that of being able to orientate a texture.  We can see that the upright wooden texture looks fine, but the crossmember texture is aligned 90 degrees to what we really wanted.

Press ESC to deselect the face.  Looks good now :)

Press ctrl+tab until you get the overhead view.  Shift+click on the blue box and the yellow box behind it.  You can actually select entities this way even through intervening regular brushes, which is quite handy.

Drag them into the room.

Press ctrl+tab and you will see the player start is buried in the ground.  Lift the entities up a notch until the player is on or just above the floor.

Press ESC and then select the player start point alone.  Press ctrl+tab to get the overhead view.  If you zoom in you will see an arrow in the blue box.  It indicates the direction the player will be looking in when he spawns.

Let's get him to face the door.  Press N.  At bottom left of the Entities window is a little cluster of boxes with degree angle numbers written in them.  Their arrangement in the box indicates the direction the number represents.  Click the 270 button.

Close the Entities window and press ESC - you can see now that the player will arrive facing the door.

Save your work, compile it and give it a little playtest.  You should see something like this:

Notice that the indoor footsteps sound is different to the snowy.  The tiles used don't actually have any special sound properties given to them (in the way that the snow texture has).  Instead ET uses a default footstep sound for anything not coming with its own sound defined - and the default is fine for this sort of texture when walked on.