If you want to add some ambiance or want to add some textured ceilings/floors to your maps, check this out.

Adding A Global Light Level

This one is simple but confusing.

You'll want to go to Map>Map Symbols and go to Plane 1 (Walls).

There you'll want to add a bunch of symbols.

You'll need to add these:

216
217
218
219
220
221
222
223

And these:

252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267

Then add one of the walls from the first table to map position (2,0) and one of the ones from the second table to map position (3,0).

The first table is for the light level (0-7) and the second one is for the visibility (0-15).

Think of it like this:

 216 - Light level 0 (Darkest)
 217 - Light level 1
 218 - Light level 2
 219 - Light level 3
 220 - Light level 4
 221 - Light level 5
 222 - Light level 6
 223 - Light level 7 (Brightest)

 252 - Visibility level 0 (Lowest Visibility)
 253 - Visibility level 1
 254 - Visibility level 2
 255 - Visibility level 3
 256 - Visibility level 4
 257 - Visibility level 5
 258 - Visibility level 6
 259 - Visibility level 7
 260 - Visibility level 8
 261 - Visibility level 9
 262 - Visibility level 10
 263 - Visibility level 11
 264 - Visibility level 12
 265 - Visibility level 13
 266 - Visibility level 14
 267 - Visibility level 15 (Highest Visibility)

Once you have all that done, add this to your map translator file:

enable lightlevels;

Adding Flats

First, you'll need the texture itself, of course. I'll be using a texture called CEMENT1 for this example.

Add this to your map translator file:

flats
{
	floor
	{
		"#FFFFFF", // Placeholder
		"CEMENT1" // Stone Floor
	}
	ceiling
	{
		"#FFFFFF", // Placeholder
		"CEMENT1" // Stone Ceiling
	}
}

Yes, you need that placeholder.

Now, go into your map and go back to Map>Map Symbols. Then go to Plane 3 (Extra).

Add the index of your flat. (eg 1 would be the first texture after the placeholder)

You'll probably want to go to Map>Plane Manager to enable showing Plane 3.

Now then, select Plane 3 as your active plane and check Map>Low Byte Mode or Map>High Byte Mode depending on whether it's a floor or ceiling.

Place some down, save and export. You're done!

Note that your textures must be inbetween F_ markers (WAD) or a flats/ folder (PK3). See this page for more info.