Head in through the gate you opened and go up the stairs. Follow the hallway around until you come to the other side of the tower. There's an elevator you can get to here, but there's a ceiling in the way.
Drag Trico's tail over to one of the holes in the elevator's floor and climb down it.
There are two rings to the room below the elevator. The inside ring, directly below, has gaps in the floor you can jump across and two doors to the outer ring. The outer ring has a bunch of suits of armor (a couple without helmets) and a bunch of mirror-shaped keyholes.
The best way to handle this is to draw the suits of armor into the inner ring one at a time. Get them to chase you into the inner ring either by getting close enough to activate them or by giving them their helmet back, jumping to the other side, then targeting them with the mirror.
When all the suits are cleared, make a circuit around the outer ring and press the mirror into each of the mirror-shaped receptacles. This opens the ceiling above the elevator. Head back up Trico's tail and take the elevator up.
The elevator delivers you into another large group of suits of armor. Use the same tactics as before. Knock down any that carry or pick up an eye shield. Yank the helmets off of any that Trico knocks down. There's a lot of them this time, so just keep at it. Use the mirror to take out the spear-throwers on the ledge above.
When the fight is over, attend to Trico, then look up at the ceiling. Point your mirror at the dot in the center to call the next elevator.
A gate locks you inside (and Trico has to ride on top), but you'll be free when you get to the room above.
Climb on Trico's back in the next room and he'll carry you up as far as he can. There's a fan at the very top that you can't do anything about.
Drop down to the support Trico is standing on. There's a small tunnel you can climb through to get to the interior of the fan where you'll come face to face with the master of the valley.
The black goo around the master will attack you like the suits of armor and if you stay too close, it will eventually kill you.
Hit the center with your mirror to drive back the goo. Your goal is to climb up the cage and up out of the room. If you continue to hit the goo, it will eventually retreat all the way inside the blue sphere, but it comes back pretty quickly.
The trick to getting past it is to stop hitting it before it retreats all the way inside the blue sphere. It takes longer to come back this way.
Drive the goo back, then jump onto the cage. Climb onto the top, then jump to the rotating fences. Keep climbing up until you can get into the room above the fan.
Hop onto the fan and work your way around to another mirror plinth. Press the mirror into it to stop the fan. Keep it held there until you see or hear Trico climb to the roof, then jump to the ledge above the fan to follow him.
Run around the roof until you find Trico.
Before you get to escape, the master of the valley fights back. The tower beams out a call to all the mind-controlled tricoes in the valley.
There's nothing you can do but watch as the reinforcements arrive.
They're not too happy with you being there. Trico takes offense to that, but he's quickly overpowered.
Stay clear of the fight as best you can and run around the roof until you see your mirror shining in the sun. Run over to grab it. It doesn't go well for you.
After a quick victory lap thanks to one of the brainwashed tricoes, you'll end up back on the white tower's roof.
Run for your mirror again. This will trigger one of the evil tricoes to pull off Trico's tail. As unpleasant as that is to watch, it give you your key to victory.
Run back to the center of the tower where Trico's tail landed. Climb down inside and find his tail on the ledge above the Master. Pick it up and drop it down into the room below.
Drop down into the room with the master of the valley and start hitting it with your mirror. When the black goo is cleared away, Trico's tail will start shooting. It might take a couple shots to get through the cage, but eventually the master will explode.
All of the connected tricoes don't handle this well and start dropping off the tower. Trico comes down and carries you back out of the tower.
Trico has a bit of a rough start, but ultimately flies you out of The Nest. On the way, you pass the only other surviving trico — the no-longer-controlled tricopath.
Trico will land briefly to swallow you again — apparently this is how they survive without pockets — and then carry you home to your village. He still needs to work on his landings, but he delivers you safely.
After a very tense few minutes, you'll realize what has to be done. With the last button you press in the game, call to Trico and tell him to go.
You've all seen Marvel movies before, so you know to stick around for the stinger after the credits.