Age of Wonders III Multiplayer Battle 21. Elf Sorcerer vs Goblin Necromancer. City Siege

I(Sorcerer) made a mistake and sent a big army to defend city which was very small and not important. Instead I should ve kept my army further in the back and reinforced it with other armies and then striked at my opponent. I also should ve built other units instead of storm sisters. But in the way I did this my opponent just rolled over me.