In a small village of Salamanca, when the temperature outside was around 30C, inside a cellar/basement it would perfectly be at 18 or even 16C depending on the corner.

Also, being underground (or semi-underground under a separate room from the house) was an advantage on wars against pillage or further bombings.

Under the Spanish right-wing dictatorship, a lot of people had banned magazines and books well kept under/inside basement trunks, relatively hidden from the usual rooms at home, as most people would just think of a basement as a room to store old junk, ripped clothes, photos, family related letters or broken hardware. And a bunch of food, maybe.

Rural Spain's homes were built on thick walls often made from stone bricks, they isolated from both heat and cold, which can drop below 0 with ease at Winter on non-coastal zones and reach up to 40C temps at Summer.