Yep, the classic large castle set.
The Mars Mission set is very similar, and is a close second but doesn't quite reach the efficiency of the original to me.
I just (finally) opened my Mars Mission set and I love a lot about it - but it didn't make me stop wanting the old space station. There's a lot of neat stuff going on here and I may pick up a module to plug in or two, and I love the lights and electronics. But it's very tight in there. But I appreciate that they're putting soda cup holders and TVs in bed. It's almost like they understand me! Minus the exercise equipment in the command center.
I remember seeing 3666 as a kid and being impressed by it. I've been sitting on a 5783 Eagle Castle for a long time (clearance, $15) that I need to assemble - never did come up with a good place to put some of the big sets. When Target used to stock more (and certain clearance outlets also got some) I bought pretty much anything that met a certain price threshold because when you're looking at a big box for about ten bucks... why wouldn't you? (Just this weekend I found a $3 sack of orangutans and a $6 70071 Robotitron with Glider. Admittedly not the greatest clearance finds of all time, but it's been a long time since I've even been out looking.)
