This unusual rooftop is located on top of a gym in down-town Madrid, and unless you know it's there you'll never find it. Unfortunately, all the locals seem to have discovered this hidden gem already and it can get jam-packed on weekends, especially when the sun is out.
The crowd is 25-35 y.o, lots of beefy guys thanks to the gym below which makes for a very entertaining people watching session. Just the kind of experience me and my girlfriends enjoy - an afternoon of sipping cocktails and watching guys flex their muscles. Very casual vibe, somewhat reminiscent of a Miami beach club, there are tanning beds and low chairs spread over a large artificial lawn. House/Lounge grooves play in the background at a volume ideal for chatter, get there early and try to score one of the large beds.
Although the terrace has been designed to be used all year round, I believe it's probably best during spring. You would want the sun, but probably not the extreme summer heat.
Lots of variety in their drinks list - many classics, a few mock-tails as well as a selection of healthy shakes. There are even some drinks mixed with isotonic beverages - you know, so you can feel a bit more like a health freak and a bit less like a drunk. Try their "Vodrojito" a distant relative of the mojito, made with red berries infused vodka, great for a hot day. There is also a selection of light snacks to go along with the health theme.