We all know that the reason why The Emperor still came to a stormcloak controlled city was due to gameplay reasons. But let's come together and make our own reasons on why Titus would still head to a now enemy controlled city and the Stormcloaks themselves would have no problem with that.

I personally, would guess that the death of his niece bothered him enough to where he doesn't care about if the city was no longer loyal to the Empire. Either that, or he had welcomed death a long time before he even knew the Dark Brotherhood was after him. And the Stormcloaks themselves probably didn't capture him our of fear of the Empire sending full military action against them for retaliation or Titus's official reason for coming was a diplomatic meeting but was really there over his niece's death. What are your reasons?

When was Solitude under Stormcloak control? In the game, the Imperial Legion is headquartered there Didn't he come for the wedding (not simply gameplay)?

Does it fall under the Stormcloaks if you side with them and finish the civil war questline? I've only ever sided with the Empire, so I don't know.

Actually, if you listen to Vittoria Vici, she complains about the Emperor not coming to her wedding.

Ah, it's been a while since I did any DB stuff.

Yeah, it does. Elisif pledges loyalty to Ulfric after you execute Tullius.

Yes. Solitude becomes Stormcloak turf once you complete the Civil War Quests as a stormcloak.

To Ulfric, it looks like a win. Let the Emperor into your walls so someone else will kill him.

Agreed. From the Emperor's point of view he would want to show the Stormcloaks a gesture of trust and would try to form an allegiance with them. If he didn't show up it would come across as a sign of weakness. Its a win/win for Ulfric.