Grepolis land units differ in weapon type, attack, defensive values, speed, population cost and loot capacity. A strong army is one suited to its job, not simply the largest army you can afford.
Match weapons and defence
Compare an attacker’s weapon with the defender’s matching defensive value. A unit may resist one weapon and be vulnerable to another. Check the current in-game unit table because world settings can affect values.
Account for transport and population
Land troops can raid cities on their island and travel farther when carried by ships. Their population competes with other units and city development, so recruitment has an economic cost.
Before a major move, use reports and the Agora simulator. Read the battle guide, defence guide and Grepolis hub.