Tower Invincible: Secrets
You can find a few bonus chest inside hidden rooms in the Tower Invincible, aswell as really well hidden room where you can meet Svetlana Twofeathers. To get all those secrets, you will need the Spike Sphere/Micro Spider Sphere, the Sonic Sphere and a Rail Hook or a way to fly.
Hidden rooms
Sewers
If you fall into the trap at the Tower main entrance, you will reach a healing water pond at some point. There, you can see an opening at the bottom of the pond: use you Sphere tech to go through it and pull the lever in the small cave.
Then head to the room above the pond and head to the right. You will find a closet with a small openingin the wall and you can use your Sphere tech to reach the sewers of the Tower. The switch to the left of the room will open a way back to the platforms exiting the healing pond room. If you head to the right of the sewers, you will find a few extra chests. Walking near the sewer tank in the middle of the room will release 3 Snauts near the chests and walking to close of the chests will release 3 Pteropods in the middle of the room and 2 (i think, they tend to stack on each other) Po Golem near the exit of the room.
Hidden room before the hostages
To open this room, you will first need to pull a hidden lever. This lever is located in the top left corner of the room you reach after the two paths to enter the Tower rejoin. Use the Spike Sphere to reach a small alcove in the roof of the room.
After pulling the lever, cross the puzzle jump room to the left and head toward the room with the hostages. On your way to that room, you will find a room opened in the wall to the right with 2 extra containers.
To reach this room, head to the last room before the boss where you face a lot of Glitch soldiers. From there, head to the room to your left and climb down the stairs. Just before reaching the Forge, you will find a small room with a bed. You can interact with the wall to the right to open a small passage. To cross this passage, you will need the Sonic Sphere tech because a laser will activate once you are midway through the passage, dealing heavy dammage until you reach the end of the passage. Charge your Sonic Sphere at the entrance and you will cross the passage unharmed, reaching a room with 4 extra containers. If you exit this room to the right, you will also be able to skip the room with all the Glitch soldiers before the boss.
Svetlana
To reach Svetlana, you will first need to pull 2 levers hidden inside the Tower. The first one is located inside the Library and the second one is found at the bottom of a lava filled well under the Forge.
Library Lever
The lever of the Library is well hidden. You will first need to activate 2 switches disguised as bookshelves.
Pulling those 2 switches will open a small passage in the room in the top right of the library, where you'll find the first lever.
Lava Lever
The second lever it found at the bottom of a well full of lava under the Forge, after the pool of lava with the hard light bridge. If you have lava immunity, you can just dive inside the lava to pull it, but there's a way to drain the lava. From the room where you find the console to activate the hard light bridge, head to the right and ride the elevator leading you below. You will reach a small pond of water, but a hole will open in the floor once you're midway through the pond dropping poison and turning the whole pool in poison. After the pool, 2 melee Glitch are await you. Behind them you will find a drill with a console. Activate the drill and head back to the the hard light bridge and head to the right. You will find the well drained from any lava and the second lever leading to Svetlana is at the bottom.
Path to Svetlana
Once both levers are activated, head back to the Dark Room above the Library. You will find an opening in the roof at the right exit of the Dark Room.
Climb through this passage until you reach the top of a tower. There you will find a rail to your left. Either use the Rail Hook to ride the rail or use some way to fly to follow it. After reaching the other tower, head downstairs and you will find a room with two propellers blocking your path. Charge your Sonic Sphere on the edge of the pit and then release it to cross the propellers. You will then reach Svetlana's room, where you'll find a good ammount of containers with high tier random loot. You can exit her place and go back to the dungeon by pulling the lever you find in her bedroom.