Duolingo might actually be almost perfect if it's your first foreign language. But I've studied whole 3 languages with it and on the fourth its flaws became too glaring to be able to cope with...

Also the majority of courses is in English. The problem with this is that for me, being familiar with several different languages, it might be unnecessarily complicating my life by starting from English instead of another, more similar language. For example, my native language is Russian, and in grammar and vocabulary it's way closer to Polish than English is. And there's only an English-Polish course... And at this point I'm not 100% convinced that English is the best entry point into Welsh, either. They say there's a lot of similarities with Italic languages (such as Spanish)... Although, of course, there's almost no chance I'm finding Spanish-Welsh resources.

By the way, have you checked out Duolingo Flashcards? It should be somewhere in Labs (or just search for it).