This is my second time to play Skyrim and the second to do Aftand but I don't remember what I have to complete to get the door to the upper tower to open. This is the tower that has a path leading up to it and it is covered by bars, but you can see the lever that opens the ruins. Can anyone let me know, please?

You have to visit Septimus Signus out on the ice north of Winterhold College, either via the MQ or Hermaeus Mora's.

That lever can only be reached from blackreach it's an easy way back in, there should be a passage going through a cave a bit lower on the cliff if you're where i think you are.

Thank you - I'll try to find the cave - I've seen Septimus...

The cave's just slightly north of the tower still part of the same ruin should be easy to find.