The Arsenal is a building that you upgrade your troops from level 10 to 11 , 11 to 12 but you have to have the appropriate building up to level first. Meaning you can not upgrade you cavalry from 10 to 11 unless you can make level 11 already. To Build the Arsenal you need :
Food : 58,000,000
Required to be keep 27 first. This building does not upgrade at this time.