Learn Italian: La collina dove abbiamo camminato ieri è indimenticabile. - The hill where we walked yesterday is unforgettable.

Yes, you can say La collina in cui abbiamo camminato..., meaning 'The hill in which we walked'. However, dove is much more common and natural in everyday spoken Italian when referring to a physical place. You could also use sulla quale (on which) if you want to be very precise about walking on the hill.

Is there a difference in tone between using dove and in cui here?

Yes, using in cui makes the sentence sound slightly more formal or written. In everyday conversation, Italians almost always prefer the relative adverb dove when referring back to a literal, physical location like la collina.

I thought verbs of motion use essere. Why is it abbiamo camminato instead of siamo camminati?

That is a very common exception! While many verbs of motion that show a destination or change of location (like andare, partire, arrivare) take essere, verbs that focus purely on the physical act or exercise of moving (like camminare for walking, passeggiare for strolling, or nuotare for swimming) take avere.

Why does indimenticabile end in -e if la collina is feminine?