Smokey (update: 5/25/08) (update: 7/23/08)
Don't be mislead by his serious expression; Smokey is both affectionate and playful (he loves tackling me by the ankles) and he loves his entry perch shown in the photo. The perch is about seven feet above ground; the "tunnel" at the right leads inside (he'll crouch in the tunnel when it's raining or snowing while taking a look around); the ramp (down center) is carpeted and leads to ground level. But, except in inclement weather, this is also his favorite vantage point, offering a prime view of his domain as well as a secure stronghold from which he can defend against all comers. (The mast behind him is my old radio mast.)

Smokey has another entrance as well; a swinging door from the porch into a "basement" compartment and, from there, another door to the stair well by the main door (human door, that is).
In so many ways, Smokey has the best of all worlds. He has a comfortable home with us but he can also walk on the wild side as he wishes (and is very good at catching mice, shrews and occasional birds). Of course, the wild side does have it's dangers and, on one occasion, he has come in pretty chewed up but it's his preferences that count. He's a companion, not a pet and we have to respect him accordingly. On the other hand, since he's pushing twenty pounds of muscle and moves like a cheetah, he's hardly defenseless either.
Oh yes, Smokey also likes to stand guard over us while we're in the hot tub at night. He'll show up as soon as we go out, sit on the lid, prowl the immediate area or jump on the roof and watch from there ... then, as soon as we go back in, he'll be there ready to curl up on the bed with us.
Update: 7/23/08 New Platform / Shelter
Smokey's platform, as you can see above, has been rather small. It was intended simply as a transition point from the ground to the ramp to the tunnel to inside ... but it's also been a favorite perch so I decided to remodel (so to speak).
His new platform is not only much larger but an old PrimeStar satellite dish has been turned into a roof (drain holes are drilled around the edges). The platform and roof are roughly the same shape and he now has about eight times the platform plus cover for inclement weather.

He's already showing a great fondness for his new perch. The roof is high enough to let him stretch but also low enough to feel like secure shelter. He has a 360º observation view, a defensible position and a line of retreat to his ultimate safe spot (i.e., inside with us).

Here you have a bit better view of the ramp (the cleats are for ease of descent, not ascent) and his tunnel entrance (a 1"x4" plank with carpet and a clear corrugated vinyl sheet bent and screwed to each side).
Philosophically, Smokey has the firm belief that the universe is out to do him a kindness and keep him amused. Of course, I'm quite certain of the same*, therefore, he's in excellent company.
* What precisely is the opposite of paranoia? You know, that sneaky feeling that the universe wishes to do one a kindness?
I've had a cat door for years, mounted in the sliding window next to the driver's seat. The "door", however, has essentially been a piece of plywood with a hole and flexible vinyl strips which a cat can force his way through. (And also a rigid vinyl pane to slide into place to "close" the door when necessary.)
The one problem has been that this door is a bit drafty (and is also right behind me when I'm working) so I decided to get a better cat door; one with draft seals. Since it has to fit inside a metal track on top, left and bottom, I had to start with two pieces of 3/4 ply (below, upper left) with an opening in the lower piece to accept a commercial cat door (below, lower left).

Above, right, you can see the finished product. The two pieces are hinged together so that they can be inserted in the top and bottom frames and then slid into the left frame before the sliding glass is closed to complete the mount. Foam weather stripping completes the job, excluding drafts around the device.
In the inset door, the frosted vinyl flap (rigid) swings both ways while a small magnet in the bottom of the flap and another in the frame hold the door in the closed position and brush weather stripping around the sides provides a draft guard. For entry/exit, Smokey simply pushes through without hesitation (unless, of course, he want to play grab with me on the inside) and a small slide locks the door when desired (such as when we spot Smokey coming up the ramp carrying dinner ... which he'd like to bring inside to play with.)