The castle looked like a perfectly normal human castle, not a demon lord castle. I've long suspected the MC's magic prowess is hugely held back by his lack of imagination. Lefi stated in the beginning that imagination is paramount to proficient magic, after all. The MC dude has very modest amount of imagination and his ability to think outside the box is even more humble. He basically has big troubles coming up with stuff on his own. For example when they were testing his elemental magic compatibility, he couldn't come up with any magic effects (spells) that weren't simulating household appliances, weapons, and such he knew from his previous life. He is the kind of guy who couldn't write a fiction short story even if his life depended on it. It's quite a pity.