The Global Trade HQ has long been a source of complaints from SimCity BuildIt users who found the trading system to be flawed and annoying. The main problems were initially believed to be the sheer level of demand overloading EA's servers as the app became the most popular SimCity game of all time as well as the fact that the timelock feature reduced players ability to easily search for items. The time limit between advertising new items has now dropped from 5 minutes down to just 1 minute making it easier and quicker to sell your items and generate Simoleons. Even better the length of each item advertising period has also been extended to 12 hours meaning that your Trade Depot items can receive much more publicity on the Global Trade HQ resulting in an increasing chance of more sales and more Simoleons. This is a great improvement that benefits all SimCity BuildIt players and hopefully there will be more nice surprises to come in future app updates!
Once you have unlocked your Cargo Dock you will begin to receive cargo shipment orders for 3 different types and numbers of items from your factories and stores in which you have 18 hours to find the required items and complete the order to receive 1 Golden Key.
Provided that you complete an order that will arrive every 6 hours then you can soon build up a decent amount of Golden Keys to use in buying your first premium buildings. The method to this madness is that for each building you rebuild you can receive up to multiple Golden Keys and since several buildings can be destroyed during each challenge this means you can quickly generate a larger number of Golden Keys much faster than by fulfilling Cargo Shipment Orders. So there you have it - the only two ways to generate Golden Keys to use in boosting your city to bigger and brighter things! Optimize your city layout from the very first day with a strategy for building your roads using as little space as possible. It is time to start the party with the SimCity BuildIt Rio Carnival,  Lunar New Year & Valentine's Day Update! Rio Carnival: Light up your streets with the Carnival Stage, towering Parrot Statue, and more! Add 17 new specialization buildings like a Mountain Lift, Winter Castle, Half Pipe and Train. Every time I play Simcity 4, my cities usually end up broke and overpolluted by the time they reach 100k citizens. Almost all of my successful cities ended up being ones surrounded by broke, over-polluted failed cities -- to which I could export all my pollution and garbage.
Actually, now that I remember it, one thing that definitely helped was planting lots and lots of trees. The utilities buildings, water pumps, power plants, and incinerators age over time, more for the more they are used.
Special note, if you are using Waste To Energy incinerators, turn them down to producing no power and have them only dispose of waste. Try using the Network AddOn Mod (NAM) to round out the types of roads you can build and increase the distance sims are willing to commute to work.
Also, Simtropolis is the single largest online community for SimCity and they have thousands of helpful users on their forums with invaluable information to help you succeed at city building (not to mention the Simtropolis Exchange (STEX), which has tens of thousands of user created buildings, models and mods you can download to enhance the game).
I've not played in ages, but I seem to remember that the trick was to not become too attached to any particular segment. But, again, you have to be pretty heavy-handed with the bulldozer (not all at once!), and willing to make constant adjustments as this or that area gets too much THIS and not enough THAT. That said, though it's been a couple years since I've picked it up, have you ever tried making a smaller city adjacent to your current one which functions as your area for dumping garbage, factories, etc. These steps would help you to build a large city with high skyscrapers for both residential and commercial zones.
However you start your town off, grow it to a certain size where it is well self-sustaining. As you begin to develop a megalopolis there are other issues that will crop up, such as intercity transport links and the failure of your dirty towns as development wants to spill over into them and demand for dirty industries decreases. One tactic I like to use in medium and large tile cities is to develop a certain area for whatever density (usually low or med). This tactic allows me to specifically tailor my civics to the exact needs of the city, and to utilise some cities' strengths to benefit the other cities. Start making your first city, the on with medium-high wealth residential, high wealth commercial services, all commercial offices and some hi tech industry by zoning some medium density residential area. When zoning the industry, don't worry too much about traffic or any fancy zoning method, as it will all be demolished shortly.

If your budget is in a strong position in this city, you might consider removing the dirty industry and switching to renewable energy, if you have unlocked solar. At this point go back to your original city and remove the industry and coal fired power plant(s), or if you have already done so remove the renewable power plants you built to replace it. Vu's Tower you are suddenly able to unleash an unlimited number of Disaster Challenges upon your city by giving Dr. This prevents the plant from aging in any noticeable amount, and you can then buy your power from a cheaper plant. In order to build a large city first plan the layout; in large cities there is always a problem of traffic.
Reduce or eliminate the dirty industry in this city so that your Sims are going over the border to work at the factories. I usually make 6x6 blocks, surrounded by road, and make a 5x5 square of these, to make 25 blocks total.
You will first need to allow development of some low wealth residential to help get the city on its feet, as well as some dirty and manufacturing industry. The reason I don't generally build too much of it in my wealthier city is because while residential and commercial towers can grow upwards, all types of industry can really only grow outwards, so the land demands in the other city would be too great. If you have unlocked nuclear this is even better but these arrangements will only be temporary so don't fret too much about it. Vu some Batteries, Launchers and Gloves each time you want to create natural disasters that destroy your buildings and require you to rebuild them.
The downside is that your power budget will not be at 100%, so 1) new plants will start at whatever the budget is set to overall, and 2) there is a chance of fire on all power lines. However, I made sure all the info relevant to this topic is copied on this page, just in case! After some years of establishment build a complete education system, and try to balance your budget. Do build parks at regular intervals, this would keep the desirability for the high zones to certain extent. Locate all the dirty stuff (coal power plant, landfill) at the far side of the tile from where the neighboring small tile is.
A good time to do this is when you are about to outgrow your water towers or they have aged and need replacing.
On the edges of this development, put all that vacant land to work with industrial development!
I put several of these squares in my cities and I use some form of higher capacity transport in between. When zoning the residential using the squares method above, I usually leave the centre block clear to place things like libraries and clinics, but don't start placing any civics until the city is on its feet and you want to start bringing in more wealthy citizens.
Make sure there is ample power and water coverage around the city, then start bringing in the medium-high wealth residents. If there is more residential demand zone some more medium density residential, if there is more commercial demand rezone some of the commercial area near the centre of your city into high density to allow development of skyscrapers. This city will be fairly self contained, but it will be critical to the survival of the other cities eventually. Instead of selling power directly from your industrial city to your wealthy city, buy plenty of excess power in the hi-tech city and ell the surplus on to the wealthy city. The trick I find is to constantly evolve your towns and cities, ensure there are plenty of good reasons for residents to want to live there, ie low crime, hospitals, education, parks, transport, plenty of water and power.
I usually use avenues for this, and zone medium density commercial next to these avenues as they will be traffic thoroughfares. Zone high density industrial in the square pattern above, again making sure there is adequate mass transit, linked up to the mass transit system of your first city.
Remember, if you can place a water pump and sell all that water to a neighbour, you will be making a profit.
Build regions within your town, so you'll have a residential area, a commercial area and industry.
Go to the small tile and make that where you concentrate your dirty industries, power generation and waste disposal.

Leverage your cities' strengths so each can profit from revenue streams that become more critical as the income from easy growth tapers off.
In particular, if you find you have a water crisis and as a result several buildings are black because they are not provided with water, they can remain that way long after you have reprovided the water.
For mass transit I generally use a subway station in the middle of these blocks, with a bus stop or two next to it and another 4-8 bus stops spread out around the block. At this point you should start zoning medium density commercial to provide your more wealthy sims with jobs, keeping the commercial areas near the centre of the map to reduce the average commute time.
You can either place a coal fired power plant in a corner somewhere or buy power from the first city - either way these arrangements are only temporary. Zone some area for high density residential and high density industrial using the same square pattern, placing high density commercial on the avenues. Ensure there are plenty of transport options so people can get around, put tolls on some of the roads to aid with the budget.
If this is a residential area there is hardly any need to build a fire station, but if required build only one at the centre. That is, have your clean city sell water to your dirty industrial city and ship its garbage there as well. This could mean that you simply have the wrong kind of zoning for the stage your city is at; demolish and rezone, or simply demolish and see what happens. To encourage more traffic flow on your avenues as opposed to local streets, you could place streets, as these are slower so less people want to travel on them, however I tend to use plenty of one way roads which just inconvenience drivers trying to take a short cut.
Taxes are very important in this city - ensure nothing grows except for high tech industry and perhaps slow low-medium wealth commercial services if you wish.
Throw in some coal-fired power plants, enough to generate power for your first city plus all the energy-intensive industry plus some excess (a good target is to be using 70 to 75% of your power capacity). Have your dirty industrial city sell cheap electricity (from burning coal and garbage) to your clean town. The reason I try to keep this city primarily medium-high wealth is because I know I will get a higher tax income from them, so I can give a full education and healthcare system to them. This city won't get a huge amount of demand upfront, so save and exit to region and go back to your first city. Have all taxes at 20% except those for low wealth residential, low and medium wealth commercial services and dirty and manufacturing industry. Make sure the transport network is strong, possibly including a connecting freeway and definitely trains (with a freight station at the shared border). Your nascent high-tech city can profit greatly from selling water, as the polluting industries need a LOT of H2O.
If it's industrial, it could be that your industrial area is ready for high-tech but it isn't going to develop unless you get rid of that unsightly abandoned factory.
Increase taxes for dirty and manufacturing industry as well as for low-medium wealth commercial services.
Use a coal fired power plant to provide this city with power (you will need many more eventually but start with one). This money can be used to get your expensive educational system on its feet without causing a death spiral for your budget. When the budget is looking ok, place a police station and some cheap forms of education (college, library and museum). From the start these cities can be targeted for high tech, or they can be developed to take advantage of spillover development from your successful first city with a growing high tech industry. By now your first city should be large and definitely high tech; start cities bordering on it, they will need their own dirty towns as well.

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