So that means the "Squire is a Tier 1 or Level 1 Unit", when it gains enough XP It will evolve into a "Tier 2 or Level 2 Unit" which is either a "Witch Hunter" or "Knight" depending on the building built.
Also, for later, when the tier are maxed out. Let's say a Tier 5 or Level 5. The unit will keep leveling, getting stronger attacks etc. but will not evolve.