What it teaches
Which words take a capital letter in Polish. Names, cities, rivers, mountains, animals' names and the first word of a title do. Months, days of the week, languages and adjectives made from country names do not.
That last group is the one worth the game. In English it is Monday, December, Polish, German car. In Polish every one of those is lower case, and a child who spends their afternoons reading English writes them the English way without ever being told to.
How it works
A sentence appears with a gap and two versions of the same word — one with a capital letter, one without. Pick one, and the rule appears whether the answer was right or wrong.
Three levels. The third brings in the awkward ones: Polak with a capital but polski without, pan small in an ordinary sentence, and Słońce capital when it means the star.
What to look for
This is klasa I and II material and it stays useful much longer, because the rule about adjectives catches out adults too.
The game is in Polish. Everything else on this site is in English on purpose; the Polish subject is not, because a spelling rule cannot be learned in translation.