To be honest, Castiel had been concerned about his brother even prior to Viktoria leaving. While he hadn't personally met Cassie, and while Castiel had once been opposed to the union of his brother and the witch because of what the outcome would bring, he knew well enough that the fact that she had come back couldn't have been easy on him. Castiel didn't push for him to talk, or even brought up the subject, but his attempt at showing him that he could talk to him if needed had been with the old bottle of whiskey that he had acquired for him during their last visit. As if the old bottle of alcohol would serve as a way to tell him, I'm sorry you had to relive losing her again.
Yes, Castiel was an angel employed by Heaven, and his brother was a fallen, but ultimately Azazeal was still his brother. Azazeal had helped Castiel when he had needed it, without a second thought or asking for anything in return, and the angel would always feel thankful for that.
It wasn't just gratitude that made him want to check on him, though. Along the way the two had ended up siding with different alliances, had once different goals in mind, but to Castiel the bond that they shared as brothers couldn't truly be broken.
When Viktoria had left, that was when Castiel couldn't keep away without checking personally on Azazeal anymore. He would never pretend to understand the bond that Azazeal and Viktoria shared, but the way that Castiel figured was that they had lived together for so long - her absence must be felt, more than Azazeal would probably ever admit. When she had asked him to check on him, it was just a stronger confirmation that he needed to see his brother, not just share messages with him over the net.
Once he arrived, the suggestion to close the door was followed instantly, and he walked over to the teapot to accept the offer of tea.
"Is there anything you miss, of the time you spent abroad?" Castiel asked as he took a sip of his tea, his eyes on the window for a moment before turning to Azazeal. "I would assume that Everett must seem so small sometimes."