Do I have to leave Strava to use this?

No. CycleLytic sits on top of Strava (or ROUVY, Suunto, Hammerhead). You connect once and keep using Strava exactly as before; CycleLytic just adds the terrain-adjusted analysis.

How does it decide what counts as hilly?

By climbing rate: vertical gain per distance (feet per mile, or metres per km). Every ride is bucketed into flat, rolling, hilly or mountain, and your speed, power and heart rate are reported per bucket so you always compare like with like.

What's free, and what's behind Pro?

The free tier covers the activity feed, full ride details and community features. Pro ($36/year) unlocks the full analytics suite: terrain stats, trends, route analysis and training zones. Your own rides’ core terrain analysis isn’t paywalled.