There is no castle and not much in terms of ruins. The English language plaques don't explain the significance of the castle much. But the views from the ruins are beautiful and they do have an interesting VR tour of what the castle would have looked like from the inside as well what the Sendai settlement would have looked like during the Edo era. It is free and worth a visit.
You can scan the QR code there and see the virtual Aoba Castle, the ruins was obvious and accompanied by the statue of Date Masamune overviewing Sendai city. If you stand a little bit away from the sides or back of the statue, you can capture a view like Date Masamune and his fellows as visitors taking photo in front of him. The road walking down to the nearest station is a bit dark during the time of sunset as it is a forest next to it. Beware of bears and watch you steps ⚠️🐻
Ruins of the Aoba castle: now also available with VR tour to get an idea of what the castle used to look like. On the grounds, there are plenty of places to have a bite, or look at the scenery (you can see as far as the ocean!).
It's quite easy to walk up to the Aoba Castle grounds. Starting from the Sendai museum, there are several signs indicating the different routes you can take. Each are around 1km long.
I was unfortunately disappointed with my visit to Sendai Castle. Hardly any elements have been restored. Therefore, there is not much to discover. There are lots of parking lots on the mountain. The shrine is quite beautiful and the view over the city is also quite good. However, the long journey on the tourist bus is not necessarily worth it.
The ruin itself was nothing spectacular. The view of Sendai was nice. It is worth to make a bus stop to have shot stop. But it is not worth to take a taxi to visit this site. It cost us 2000 Yen one way from city because the main entry was closed.