So, tell me about Cave Houses

OK, they're a lot like ordinary houses. All the usual facilities: water, plumbing, electricity, doors, floors, ceilings. Is that what you mean?
Sounds a bit primitive, like cavemen.
Not really - it's just another way to build a place to live. Instead of taking a piece of land and putting up walls and a roof, you excavate (excaveate, geddit?) into a hillside and form the building by shaping the holes you dig. There are certain engineering constraints which means the rooms tend to be tubular or arched, but apart from that it's a very versatile way of creating a living space.
But aren't the walls just earth?
Generally the rock these houses are dug from is quite stable. So the walls and ceiling are self-supporting. You can line the walls with concrete (as my house is) or you can use breeze-blocks, but most people just leave the walls of rooms as rock - generally painted white.
Isn't it a bit claustrophobic, living inside a hill
No more that in any other room. You tend to forget that there's 20 feet of rock above your head and just treat the space as ordinary rooms. Most spanish houses have quite small windows, anyway, to keep the heat of summer out, so a cave house follows that style. Obviously only the rooms at the front of the house have windows and those tend to be the communal, living, rooms. Bedrooms and such are usually at the back - furthest into the hillside.
What about heating? Isn't all that rock cold?
Since all the walls are several feet thick, they act as very good insulation. So yes, you do have to heat the place and there are myths about cave-houses maintaining a "natural" temperature all year round (they don't), but the surrounding rock keeps the heat in. What's better is that during the summer when it can reach 40°C outside, the rooms remain gorgeously cool which means it's comfortable to sleep and you don't need air conditioning, so it's cheap, too.
I still have the impression of poky little rooms dug out of holes in the ground
To give you an example. my cave house is about 100m² which is about 1000 square feet. Since it's all on one level (you can get cave houses with multiple stories and stairs, but they're unusual) all the space is usable. Compare that with my 3-bed semi in England. Each floor is about 40m² and you lose space for the staircase and landing, so the cave house is about one-third bigger - and mine is one of the smaller ones. In times gone by (before planning regulations) if you needed more space you just got your shovel out ...
What do you like best about them?
Probably that they're so quiet. With 5-foot thick walls, once you're inside and nicely tucked up in bed there's not a sound. When I first moved in I bought a wall-clock for the hall. I had to change it as the tick-tick-tick could be heard in my bedroom and it started to bug me. I do find that for the first day or two after I arrive on a visit, I sleep really late into the day. That's because of the peace and also, with the bedrooms being at the back. the sunlight doesn't wake me either.
What changes would you make?
Well, for my particular house ... I'd probably raise the height of the doorways. The native spaniards are not very tall and they built for their height. So the frames for the doors are quite low. I have to remember to "duck" every time I go through a door. That got quite painful when I first bought the place but I'm used to it now. Sadly, that doesn't go for visitors (at least for ones who don't have a cave-house) so you might bump your head if you come to visit.