What is a beautiful building?

I didn't say that the Persian architecture is better than Europeans, I say what is a beautiful building?

For example this has nothing special in architecture but I think is a beautiful building:

No slavery in Europe, give me a break.

Before 1400: Slavery had existed in Europe from Classical times and did not disappear with the collapse of the Roman Empire. Slaves remained common in Europe throughout the early medieval period. However, slavery became increasingly uncommon in Northern Europe and, by the 11th and 12th centuries, had been effectively abolished in the North. In Southern and Eastern Europe, slavery remained a normal part of the society and economy and trade across the Mediterranean and the Atlantic seaboard meant that African slaves began to appear in Italy, Spain, Southern France, and Portugal well before the discovery of the New World in 1492. From about the 8th century onwards, an Arab-run slave trade also flourished, with much of this activity taking place in East Africa, Arabia, and the Indian Ocean. In addition, many African societies themselves had forms of slavery, although these differed considerably, both from each other and from the European and Arabic forms.

Oh...ooops

Isn't the object to make the visitor humble in front of the massive cathedral/mosque? I get awestruck when I see either of them, even though I'm neither muslim nor christian.