So now we have a house with no doors and no windows. Let's split one of these brushes and provide an entrance to our lovely home. The Clipping Tool will clip (cut) parts of a brush off, or split a brush into two sections; the icon resembles a cube with one of the edges cut or chamfered off. You can also press Shift + X to select the Clipping Tool. You need to select the brush you want to clip before you select the Clipping Tool because you cannot select an object with the Clipping Tool selected! Select one of the long walls in our house and then select the Clipping tool. Drag a vertical line in the middle of the brush where you want it to be split. The clip line is defined by two white handles connected by a light blue line. The clip line extends along the entire selection so it's not necessary to perfectly align the handles with the edges of an object, but it is good practice because it will help prevent off-grid vertices.

Notice that one half of the brush is red and the other half is white. When you press Enter, the red half will be deleted and the white half will remain. Since we want to just split the wall without deleting anything, keep pressing the Clipping Tool icon (or the keyboard shortcut Shift + X) until both halves of our wall are white. Then press Enter to cut the wall in half.

A quick way to make a door in a wall is to clip the wall vertically twice, and drag the bottom of the middle brush up to where you want the top of the door to be.

Using this method, create a door 64 units wide and 112 units tall. Use the player start entity as a reference scale. Since the wall height is 128 units, the brush at the top of the door will be 16 units tall and 64 units wide.

There you go! You now have a way to enter and exit your house. Why don't you give the map another test compile and check out your handiwork.