ok i did it on the side,

https://uk.geocities.com/cook_the_kids/tutorialthing.zip

its prety simply, just boolean a from b (select the cylinder first then the object which will create the hole) the first one (closest to middle of grid) is just the basic shapes.

2 = combine inner and outer cylinder then select the cylinder and then the box then boolean

3 = deleted the extra polys stopping you from seeing inside

4 = 3 but with a cube as a door

5 = 3+the 2 combined cylinders, then boolean one from the other to get a curved door.

6 = cylinders with curved door

hope this helps