You'll notice on some maps hilly areas, in certain places, the
only map using it that I can currently think of is dust, and its
barely noticeable, but its there. On say a map with sand, at the
corners if you look and its all flat it looks a bit odd, however
fill it up with a little sand and it looks great. Its hard to
explain, basically I mean like this pic (fig1.0):
(Fig 1.0)
You see in the corner it rises up. Now it took me a while to figure
out how to do this since then its been a great way of making my
maps less flat.
Now
first off I suggest you make a box shaped area to put your corner
piece in, like in my pic there (fig1)
Now
once you have that all ready made its time to start. First off
you need to put a block in a corner, like in pic (fig1.1) below

Fig
1.1)
Its
just an everyday block, now click the vertex tool and the dots
as in the picture (fig1.2) will appear. Now just go into top view,
and move a dot so it makes a triangle like in the picture below,
when it says merge verticals, click yes:
(Fig
1.2)
Now
select your object again and click the vertex tool again. It will
turn into the pic below, however you can see ive clicked the red
dot at the top left in the picture below (fig1.3) well I clicked
on this by using the camera, yes the camera! click into the camera
view and click only

(Fig
1.3)
Now
move this point down in front view, move it down to the floor,
straight down, dont go off to one side or anything, when it says
merge verticals, click yes, If you've done it right it will now
look like fig 1.4 below
(Fig 1.4)
Now
we need to do exactly the same for the other side, you can just
make out in the picture below (fig1.5) the top right dot has now
been clicked in camera view. Move this dot exactly straight down,
keep to the grid. and when it says merge verticals, click yes
(fig
1.5)
Now you will have a corner mound as in the picture below (fig1.6),
ive found that varying these steps results in errors, i.e. if
you make a wedge and turn rotate it rather than making the block
at the start, it gives errors. if you click on the toolbar and
click map, then click check for problems, it should pop up saying
no errors found, unless you didn't pop in a info player start
:P (which doesn't affect the mound in any way by the way :)
(fig 1.6)
Now it doesn't quite look like a mound at the moment, but you
can resize it as you see fit, just select in top view and move
as you would to make any object larger, and use the other views
for the height as you would for any normally object, always keep
to the grid. do not vertex it again as well, use normal resize
methods.

Well
there we go, the final mound in action there. Looks much better
than a flat corner I think, looks great with sand, although I
used some rather horrible textures from the halflife wad, so when
you download the example it dont give wad errors. however one
final thing, aligning textures on the mounds can prove tricky,
it seems best to click the floor with the texture
application, and click lift and select, then change the texture
app to apply texture and values, then click on the mound, you
can try click the mound on select and ticking either world or
face box, but this doesn't always align, its better to use the
other method. On rare occasions ive had textures impossible to
line up, I know that sounds stupid but it sometimes happens.