The order of constructing facilities
The order in which you must construct the facilities on the Avenger is mainly optional (as every player likes a different approach to the game), but there are a number of facilities that you should construct as quickly as possible. Those are: Power Relay, Guerilla Tactics School and Resistance Comms. To construct Resistance Comms you will need to complete the Resistance Communications research, which is why that should be your priority.
Power Relay will allow you to power up another rooms, Resistance Comms will increase the overall number of contacts, which will make establishing connections with the resistance a lot easier, and in turn increase your influence zone and supplies given to you by the Council. Guerilla Tactics School gives you access to extremely important upgrades to your soldiers (the most important being the ability to upgrade your team size to 5, and later on to 6), as well as giving you the chance to train a rookie into a specified soldier class - the training takes some time (but it can be reduced to 5 days with an engineer), but the effects will quickly become apparent, when you will start accumulating those classes that suit your needs.
Additionally, you should construct the Workshop as well, which allows you to use 2 (and after upgrading 4) drones, working similarly to normal engineers - you can use them to increase the effectiveness of nearby buildings. There are two "catches" to that - you can only empower the facilities that are adjacent to the workshop (the one above, below and to the right and left of it), and you will have to put 1 or 2 (for 2 or 4 drones respectively) engineers inside for the facility to operate. The effect is certainly worth the effort - for 2 engineers you will be given 4 drones, doubling the effectiveness of the said staff members.
Where to place your facilities
The way you construct the facilities, especially in the case of the first ones, is an important matter - proper planning will allow you to use the limited space to its fullest.
Imagine that your base (the space for facilities) is divided into columns (3 - A, B and C) and rows (4 - 1, 2, 3 and 4), and then do the following:
- Build the Power Relay in the A1 field - additional power will quickly come in handy, and without the sufficient amount of it your development will quickly come to a halt.
- Build the Guerilla Tactics School in the A2 field - you will be able to increase the size of your team practically from the very beginning, and after only a few missions you will be able to reach 6-man squad, tremendously increasing your chances of success on the battlefield.
- The A3 field can be used for anything - it's not mandatory at the moment.
- Build the Workshop in the B2 field - and with time upgrade it in order to increase the amount of drones you can use.
- Fields B1, B3 and C2 must be used for Resistance Comms - thanks to the Workshop from the B2 you will be able to fill 4 spots in the Resistance Comms with the help of drones, saving on those precious engineers early on, and the high amount of available contacts is your top priority.
It's quite obvious that you won't be able to construct all of that from the scratch (especially the 3 Resistance Comms facilities), but it is definitely the plan that you must seek to complete - the more contacts you have, the more supplies will be available to you, as well as allowing you to counteract the AVATAR Project more efficiently. This should be your priority and you should never allow a situation in which you don't have a free contact to establish connection with the resistance.
When you construct all the buildings from the above bullet points, you can focus on further development of your base - before you do so, however, don't even think about constructing anything else, as you can quickly lose the whole campaign if you won't be able to destroy alien facilities appearing on distant continents.
What 's next?
Further development and what you construct first is quite optional - it will also depend on the research you focus on. You should, however, construct the Psi Lab early in the game, as it will allow you to unlock the fifth soldier class, the Psi Operative. It is important because of the fact that the Psi Operative does not receive experience points like other classes do (from killing enemies) and must be placed in the Psi Lab for further training, which takes quite a lot of time to complete. The sooner you construct the Psi Lab, the quicker you will gain access to more powerful and numerous psi abilities.