When fighting in the arena against another player, neutral creatures will appear from a crypt in the middle that will attack both sides. Send your creatures at the same time as the neutral creatures towards your opponent to increase the force they have to deal with.
Battles
Your enemies will be resistant to physical or magical damage. Use Arrow Towers against enemies without physical resistance, and Magic Towers for enemies without magical resistances. Place those towers in range of Barracks Towers, which will summon guards. Your enemies will automatically fight the guards, giving your towers more time to damage them. You can change the position of guards to have them defend better.
Gold
Complete the achievements to earn extra gold and stone. You have to manually accept each reward when completed in order to add the gold and stone to your total. Complete the daily mission to earn 500 gold and stone.
Upgrades
Start by upgrading your Archer or Magic Tower to prevent early losses. Then, upgrade Quarries to increase the amount of stone you can produced. It is required to upgrade your City Center and attack towers. Stone will be more important as gold. Gold is only used to unlock new creatures in the barracks, new spells in the temple and upgrading your quarries.