Below is an image showing the prerequisites for each HQ level. Your primary focus for construction should always be to upgrading your HQ and the prerequisites for upgrading your HQ. I recommend purchasing the second and third construction queues if you plan to play this game long term. They will greatly speed up your leveling process.
With 3 construction queues, you can level up your Tech center plus the current and next prerequisite for your HQ. For instance, when you reach HQ level 24, you can start the Tech center to 24, the Tank center to 24, and the Hospital to 24. Use speed ups during Arms Race and VS on your 2 current prerequisites but not the next one, i.e., use speed ups on Tech and Tank centers to 24 but not the Hospital 24 as you got time to level that.
Once you complete your prerequisites for HQ 25, then you can start construction on the HQ 25 and continue construction of the Hospital to 24 in preparation of reaching HQ 25. The third construction can then go to a other things until the HQ construction completes.
Beyond HQ construction, you need your gear factory at 20 so you may upgrade your gold hero gear beyond level 40.
Your iron, coin, and food vaults should only be leveled for VS and Arms Race points.
Your training centers, component factory, smelters, material workshops, iron mines, farms, and gold mines should be leveled to 20-25. Beyond that level them for AR and VS points.
First hospital needs to be leveled to 25 but the other 2 can stop at 15-20. You can increase hospital capacity better with Tech Research.
Squad pads and tavern should be leveled 15-20.
Builder’s hut is debatable but 20-25 is reasonable.
First tech center, tank center, and one drill grounds need to keep up with your HQ. Second tech center and other drill grounds should be 15-20. Aircraft and Missile centers should be leveled up when you have openings available as they strongly increase your power depending upon your squad make-up.
Alliance center needs to be leveled up to 23.
Barracks are the most complicated (See Arms Race section). The first one should be leveled up with your base for maximum level troops (for example, 27 for HQ 27). A second barracks at one troop level below maximum (continuing the same example, 26 for HQ 27). The third barracks should stop two levels below maximum troop level (23 for HQ 27). The fourth barracks should stop three levels below maximum troop level (19 for HQ 27).
When I say “First” I mean the structure with the highest level survivors. When you quick deploy, it will assign the gold and purple level survivors to the “First” one, the purple and blue to the second one, etc.
Your alert tower and recon planes only take a few seconds to level and should be kept at maximum.
As you level past HQ 20 your rss requirements skyrocket so its important to conserve them to focus on leveling your HQ.
Exceptions to the recommendations above would be to level things up for Arms Race and VS points.
