You will be awarded briefcases when you reach certain building milestones, as well as randomly while collecting money. You will usually get them when you watch an advertisement to start the Carnival or Enable or Extend deliveries. Each briefcase contains three random cards that result in a brand new building, a building that you already own, or one of the three types of currency.
Building cards
You can examine your Building collection in the "Briefcases" menu. There are 25 buildings you can collect and upgrade. When you have the required numbers of the same building card, that building will instantly level and give you more profit.
Deliveries
Enable deliveries to double the amount of money earned from all your buildings for two hours. You can also add an extra two hours to a maximum amount of twelve by watching additional advertising videos. Watching the ads is the fastest way to get extra free briefcases.
Love
Love (the hearts) are very useful. Love boosts your profits if you have enough of it. Love upgrades get progressively more expensive with each purchase. They are also very useful as each new purchase doubles the profit bonus you get for each heart you have. You can earn love by switching buildings between them, from briefcases, or in gifts that your friends send. The easiest and fastest way to get hearts is by switching buildings between them. Do not upgrade your Love as soon as you have the required number of hearts, because you will not gain much. Only upgrade when it is most convenient in terms of gains. Connect to Facebook and add friends to gain hearts. Each time you switch a building in your city with one in your inventory, you will earn a specific amount of Hearts. After switching buildings, you will start from the beginning. You will have to go through the Power and Construction upgrades again, but the bonus in profits you get for all your buildings will help you progress faster. Ensure that you have enough money saved before making the switch to unlock at least the first Power level so you do not have to tap to collect profits.
Upgrades
Invest in increasing the energy level first, which lengthens money production times but also improves the maximum amount that can be stored when you are not in-game. If your Construction upgrades are too high of a level compared to the Power level, revenue will no longer accrue and you will have to collect it manually each time when the money is produced. When upgrading the Power Levels, note the small number in the upper corner of the building that is being upgraded. If those numbers are similar or higher than the number shown in the same location when upgrading the Construction levels, then you are safe. Get your Power to a new level first, then add ten upgrades. Follow this by upgrading the Construction levels all the way to the very end, but before switching to a new level. Next, catch up with the Power level and increase it and upgrade ten times afterwards, before repeating the entire process.
Briefcases...