Here's a quick guide to the Return to Foundation mission. This is a challenging mission, even on Easy difficulty, and you'll likely need to try it several times to get past it.
For this guide, we'll show you how we played through it on Easy. The strategy for Medium and Hard is roughly similar but you will likely need to do a lot of work with your timing to adjust it to achieve just the right balance for those modes.
First, we build an Extractor on every one of the mines.
Next, build a Gun Turret as shown just northeast of your Capitol, since that's where the enemy will come from first.
Then, we'll want to focus on upgrading our existing (non-decayed) Skyscrapers a few times, to make sure we get an immediate boost to our income rate.
As you can see, in the image above, I've upgraded each Skyscraper once.
Next, we build 3 additional turrets, one Gun Turret east of the Capitol to cover the northeast and southeast passages, and one Turret and one Missile Launcher to the southwest to cover that passage, which is the second entry the red player will attack.
Now, we continue to expand each of the four corridors separately, while also building new Skyscrapers and upgrading them frequently while we can, grabbing the income advantage before the red player has built up enough to seriously attack us.
Here, we've continued expanding to the northwest and northeast ...
We keep upgrading, and keep expanding, while starting to build military units we can upgrade later, as our crystal income begins to seriously ramp up. In this case, we're building Gunships, but a combination of Crusaders, Shield Generators, and Guardians can also work well.
We continue expanding. Note that the reason we split off separately is due to the fact that the red player will send so many units along each of the four channels, and we'd likely be overwhelmed in the late game if we were to combine any two channels into one.
More upgrading, more Gunships ...
We have a solid income lead over the red player; it's time to build quite a few more Skyscrapers and lock in that income lead still further, and then upgrade our air force as fast as we can.
Note that even before it's fully built, it can be tricky to finish off the red player's base. Don't hesitate to use Mothership effects as needed. In particular, it's often helpful to use Radar Sweep just before your units reach the enemy base. The moment it wears off, use Heal to repair those same units while casting Stun on as many enemy towers as we can fit inside the circle.