The map works, and there's monsters. Cool! But we're missing something, as the whole map looks pretty bland and uniform. Sector effects include lighting, damage from slime, secrets, and other nifty things.
Let's start with brightness levels. Head over into the sectors mode (mode -> sector mode, or press S), and right click your first sector (the one with the player start). You should get something like this:
Immediately on the right hand side, you should see the applied floor/ceiling texture that you've chosen from earlier. You can left click on these if you'd like to make some texture changes without having to dive into 3D mode. Beside it are the numerical representations of the ceiling/floor height, again, if you wish to do some editing without 3D mode.
Under effects, you can set your special from the drop down, or click the little square beside it to expand the menu. The default option is 0, which represents a normal, untouched sector.
From here you can select an array of special effects for your sector, from blinking lights or damaging slime floors. #9 marks a secret that counts toward your final total when you finish the level. Effects #10 and #14 involve a timed door from the beginning of the map, and can be used for special secrets or rewarding players for playing aggressively through a map.
Under "generalized effects", you can customize both damaging floors and lights to take place at the same time, rather than limiting yourself to just one. Closing the menu, you'll notice a brightness box.
The brightness determines the light level of a specific sector, defaulted at 192. Higher values, up to a maximum of 255, should be used for contrasting lights or skyboxes, while lower values down to 0 are used for shadows and dark hallways. Since there are lamps in our starting room, it's alright to leave it at the default, but in our hallway with the cacodemons, there is no light source. I'm going to go and turn down the brightness to 160.
Right click on the sector, and either type in 160 or press the down arrow twice. You should notice that Doom Builder darkened the room to match the new brightness that we've applied. This works for 3D mode too.
The key idea in creating lights is to apply brightness level contrasts in dark sectors. So, it's easy to spot sectors of 255 brightness in a dark room, but 192/180 won't be noticeable. We're going to add some lights. Proceed to linedefs mode and draw yourself two squares. These will represent our lights.
By now, you should remember that each sector has its own textures and effects. We'll apply light textures to each square. You can use either 3D mode or simply click and right click. I used TLITE6_6 for both ceilings to make them match. I then raised the ceiling by 8px so that they don't look like they're smooth on the ceiling.
Remember to texture those missing areas! I used METAL, which is a great texture for filling in borders and gaps.
But lights aren't lights if they're not bright, right? Right click on the sectors and change their brightnesses to 255 for good contrast.
From here you could add some light blinking effects using the menu from earlier or just finish up.
Then, apply a slime damaging effect to the sector with the baron of hell. Don't forget to leave a radiation suit (under things -> powerups) for the player.
For the last section in this introduction to Doom mapping, I'll briefly cover linedefs and specific actions that are associated with them, such as doors, lifts, teleports and exits.