To upgrade your structures in Empire.io, you need to allocate resources such as gold, wood, and stone. These resources can be collected from mines or purchased from the market. Once you have enough resources, select the structure you wish to upgrade and choose the 'Upgrade' option.

In Empire.io, you can research various technologies that will enhance your defenses, improve your army's efficiency, and unlock new units. Technologies range from basic improvements like stronger walls and faster archers to more advanced options like siege weapons and magical enhancements.

Yes, you can form alliances with other players in Empire.io. Alliances allow you to collaborate with fellow commanders to share resources, coordinate attacks, and defend against common threats. Joining an alliance can significantly strengthen your position in the game.

Training an army in Empire.io involves constructing barracks and hiring soldiers. You can train different types of units, each with unique strengths and weaknesses. Make sure to balance your forces for effective combat against zombies and rival factions.

The main objectives in Empire.io include protecting your city from zombie hordes and rival human factions, expanding your territory, upgrading your structures, researching new technologies, and forming alliances with other players. Your goal is to become the ultimate commander by strategically managing your kingdom's growth and defense.

In Empire.io, the tower defense aspect involves strategically placing defensive structures around your city to repel invaders. Towers can shoot at approaching enemies, while traps and walls can slow them down. Proper placement and upgrading of these defenses are crucial for survival.

While there may be some constraints based on your current resources and level, Empire.io offers significant freedom in city building. As you progress, you'll unlock new areas and have more opportunities to expand your city and fortifications.

Resource management is extremely important in Empire.io. Efficiently gathering, storing, and allocating resources like gold, wood, and stone will determine the success of your city's growth and defense. Poor resource management can lead to weakened defenses and slower progression.