here is one reason people have given up on mapping, they don't know how to use sectors to make there map work for them.  The more detailed your map gets, the more apt you are to have the game crash when loading your map.   Sectors break down the map into smaller parts, which makes handling the detail easier.  But yet, sectors can have some nasty effects if not used right.

If there are two sectors in your map, they need to touch eachother but not overlap.   If they overlap the player will not be able to move into the next sector of the map, and if they do not touch, you will get a hall of mirrors effect. If a sector lines up with a wall, again you get a hall of mirrors effect. and sometimes with doors you can end up making doorways that can not be passed threw.

The best thing to do with sectors when working with large maps is make the sectors touch eachother inside a wall.  Sense our room is only one little area we shouldn't have any of these problems.  We are going to encase our mini level in one sector sense there isn't much detail to it.

so, Click once in the X-Y window, and press ESC to make sure nothing is selected.   Now we need to draw another block aroun our little room. So hold down the left mouse button and drag a block around our little room..

now we need to move to the Z Screen and Strech the box so that it is one block above our room and one block below our room.  You should have something like this when you are done.

ok, well this block by itself just isn't going to work so we need to make it into a sector.  While the block is still selected Right click in the X-Y Screen and click Sector.  This will transform our little box into a sector.  easy enough eh?

your red box should now have the word sector on it, press ESC at this point and it will turn into a green box.

Now with that done were ready to test our map :) whoo hoo! already?! yep, lets move on to the next tutorial so we can learn about compiling and testing the map... dont' forget to save your map at this point :)