When you arrive in Aranda de Duero, the first thing you'll probably do is go for a walk around the historic center. As you ramble leisurely through the little streets, you'll suddenly come upon this imposing and grandiosely ornate church behind the main square.
At night, it's impressive. It's ground-lit and the shadows over the thousands of little sculptures and multiple Gothic archways is magical. When I visited, the interior was being renovated (which, I suppose, is for the best), but we could still admire the incredible facade, the intricate detail of all the adorning sculptures, and the curious Gothic spires on the left end of the cathedral.
During your time in Aranda de Duero, I'm sure the cathedral will be a guiding point as you explore the city, and it should be at the top of your list of things to see.