Necropolis or Castle (Which one is stronger?)

You're talking about the early stages of development, but the starting monsters in the Swamp faction are actually quite weak, and their strength lies in Gorgons, Basilisks, and Hydras. As for Goblins and Lizards, they have the weakest ranged attackers in the game; only Snakes are decent, and even they need to be upgraded. Humans have 10 HP Pikemen and 25 HP Griffins that attack twice, and upgrading benefits all units. Necromancers already have to pay a high price for the ability to create an army from corpses. However, I agree that the outcome of a battle is often decided by the units involved.