Also, I'm not sure if converting between 5 digit numbers and words is a good starting task, unless you want to dive right in with German's (in)famous word chaining ability.

Quick feedback: the website looks very polished and intuitive. I especially liked the test about articles, where I didn’t have to type. I liked that the website works well on mobile too. The content is not what I’d call games though; based on the name I expected something different than test questions and quizzes.

Some German natives may argue that the time short forms are wrong as they prefer "dreiviertel" instead of "viertel vor".