How to progress
I finished the French tree and enjoy Duolingo very much. I see that there are many more levels, and I keep doing strengthening exercises, timed and untimed. Is this the way to 'level up' in Duolingo? Just keep doing exercises? There does not seem to be another option. But maybe that is specific to the French tree.
[I am also trying out other web sites, but I really like Duolingo]
The Duolingo levels are based only on the number of XP (points) you have and are not an indication of proficiency or ability level. You can get more XP by either redoing lessons, strengthening a skill, or using the general strengthen. You will get more XP for timed practice than without.
If you check your profile, you'll see how many XP you need to get to the next level in each language. As your level increases, the number of XP you need to get to the next level also increases.