When you start the game, making your city strong and productive is often the hardest part. This is to help guide you in your journey.
Turrets are expensive on the steel, especially when you are trying to build an army, but buy them anytime you can. Place the first four at the corners of the Command Center- they protect what is most valuable to you. Bunkers make a good alternative, being cheaper, but they require soldiers to work, and take four times the space. Be careful not to have high range turrets at the edge of your city- they may bring in enemies rather than simply defending. Surround your Command Center, but don't abandon your borders. Place turrets at every opening and corner. Your army may be great, but turrets are the first, and last, line of defense. Never have a lone turret- group them within each other's firing range. Turrets from level one to four need this especially, as well as a wall to protect them. Different turret levels also have different effects. Use them to your advantage. Also, turrets range is only effective at about half their supposed range. An enemy moving at the edge of a turret's range will not likely be hit.
Turret I- Machine Gun. Cheap and inaccurate.
Turret II-Machine Gun. Cheap, Slightly more accurate.
Turret III-Machine Gun. Very good for basic defense.
Turret IV- Homing Missile. Knockback. Slow, but powerful.
Turret V- Artillery. Large knockback. Slow, very powerful, high range. Somewhat ineffective against aircraft.
Turret VI- Double Machine Gun- Fast, strong, high range and spread. Expensive, but very good.
Walls are somewhat effective, yet expensive. It is better to use your buildings at the edge of a city for defense. Most buildings, excluding walls, turrets, bunkers, and fortresses, cannot be permanently destroyed. This saves space and resources. If walls are needed, Wall I will do the jobs nicely. The higher level walls only have more health. Walls and buildings tend to be ignored, just walked around. Use this to your advantage, making a long and winding pathway to an opening, giving you more time to counter.
Though convenient, it is a bad idea to leave multiple openings in your city. Have only one until you have mechs to defend two or more. Generally, two mechs per opening, and turrets lining the path, ought to be good. Have only as many openings as you can defend at once. Some enemies may target all openings simultaneously, so you will need to be prepared.
Only have the buildings you use regularly in your city. The rest, keep in storage until needed. Buy all buildings you can get, especially troop buildings, but only use the ones you need.
Troops are easy to forget. Never have high value units alone. If possible, keep them in your city. Often times, players leave their army away from their city, allowing an invader to easily crush a defenseless city. Keep only the most up to date units. If you still have Soldier Is when you have the ability to make Soldier IIIs, you need to kill off your weaker units. Make sure you have units you can handle. If your preference is air units, build primarily air units. Keep all unused in storage. You may need them later.
Alliances are lifesavers. They bring money and units to you when you need help. Join one as fast as possible. This is the part where I try to solicit members with an advertisement. Join the Adamantian League. It is Six members strong currently, and it has high level members. No level requirement, just join. You will recieve help and protection.