Katakana was invented during Heianperiod (794-1185) in Kyoto. It is a kind of syllabic letters which was developed from Kanji. Katakana was used for stiff style document such as government official document. Actually it still uses for legal expression and law.

The form of katakana did not change since it was invented. So there is no difficulty to read official documents at that time. Of course the expression and terms are different but it is much easier.

Katakana as its shape uses for stone monument letters, too.

Normally, Katakana uses to write a word of foreign origin.