Built in the 15th century by Italian architects like Pietro Antonio Solari and Marco Ruffo, it 2 a fortress, palace, and cathedral all in one, with red brick walls rising up to 19 meters high.

1. The word 'become' fits grammatically and contextually.
2. The sentence describes the evolution or formation of the Kremlin into a fortress, palace, and cathedral.
3. Thus, 'become' is the appropriate word.