How can I unlock Dwemer doors?
Throughout Skyrim, there are tower-like structures with four fenced gates inside: in the middle of them, there are four gears and a level.
I couldn't figure out how to open these doors: what do I need to do to open them?
They cannot be opened from the outside. They can be opened by accessing them from below - specifically from Blackreach. You can get there via Alftand Glacial Ruins, and will need to do so for a portion of the main quest.