They should be... It's an open world game. It would be weird if cities weren't fully explorable if the whole landscape surrounding them is... The city Lestallum (you can spot it in the distance in the Duscae demo) for example is confirmed to be freely explorable:
Here a video of the city in a still unfinished state:
I really like it's Havanna inspired look. And even though it's not high fantasy in style it looks already really interesting and impressive.
But regarding a huge city like Altissia...I don't even dare to hope this will be all explorable, lol. XD