Castello Doria in Vernazza is a medieval fortress situated atop a rocky promontory. The castle was used as a defense to the town.
It's possible to visit the castle, adult ticket is €2, so it's very affordable. It doesn't take long to visit and you can go at the top of the tower and enjoy a unique perspective of Vernazza.
While the view is great, you find pinned my favourite viewpoints in Vernazza.
The castle is actually on your way to another stunning viewpoint, the one in Sentiero Azzurro, so it's a perfect stop.