Who built it?

Certainly the Spaniards

The people of the Viceroyalty of Nuevo España, Viceroyalty of New Spain, Spanish Virreinato de Nueva España, the first of the four viceroyalties that Spain created to govern its conquered lands in the New World. Established in 1535, it initially included all land north of the Isthmus of Panama under Spanish control. This later came to include upper and lower California, the area that is now the central and southwestern portion of the United States, and territory eastward along the Gulf of Mexico to Florida. The Viceroyalty of New Spain was also charged with governing Spain’s Caribbean possessions. Later, in 1565, the newly conquered Philippines were placed under the jurisdiction of New Spain.