While studying the basic units, Jack noticed some of them had upgrade options.
The light infantry was pikeman units. The information said that this unit was effective against cavalry units. When Jack clicked the upgrade option, it said that the pikeman could be upgraded to halberdier. Halberdier boasted stronger offense and higher defense-piercing property. It cost 5,000 coins to upgrade and ten days to rearm the current soldier.
Jack proceeded with this upgrade, along with the other basic units that could be upgraded. Spending another round of massive gold coins.
"Why are there units that can be upgraded and some that can't?" Jack asked.
"The ones that can are because this kingdom had completed the research on the higher grades of those units," Peniel answered. "The ones that have no upgrade option either have reached their best form or the research to the upgraded form is not yet done."