Information below may contain spoilers. It’s segregated by books. Click the book name to read details of appearance and history described in different books.
The following is a direct quote because I can’t imagine saying it better than Kate has:
Like an ethereal castle born from a mirage among the shifting desert sands, the People’s HQ towered above the city. Alabaster-white in daytime, at night its walls glowed with gold and indigo, illuminated by powerful electric lamps or feylanterns.
The People had made some modifications. A total of eight slim minarets, instead of the original four, flanked the central domed building. High walls enclosed the complex, puncuated by blocky guard towers, equipped with howitzers and sorcerous ballistae. Solemn guards and occasional vampires patrolled the textured parapets. The place oozed necromantic magic. (p115)
In addition, there is a statue of Shiva as Nataraja, Lord of Dance, standing in a circular fountain just before the entrance. (p115-116)
The Casino was built after the People purchased the lot where the Georgia World Congress Center stood before the magic destroyed it. They built it in five years, and the main floor is indeed a casino. (p114-115)