Heres a small chart of the Specialties of each type of unit.

Archers: Ranged unit, below average armour, average power, average hit points, average speed
Calvary: Close-range unit, above average armour, above average power, above average hit points, above average speed
siege weapons: Ranged/close range unit, above average armour, above average power(rams to building), above average hit points, below average speed
Infantry: Close-range unit, average armour, average power, average hit points, average speed
Ships: Ranged unit/Close-Ranged, average amour, average armour(Cannons Above Average), above average hit points, above average speed

(Towers are pusposely not added here)