Timestamps:
Clash at Castle Dedede 1:58
Echo's Edge 6:06 / 6:40 (Door appears)
Nature's Navel 9:05
Planet Earthfall 10:50
Planet Caverna 13:01 / 13:15 (Door entered)
Ability Planet "Not quite a secret room" 21:27 / 30:47 (Rewards)

I love these types of secrets. They really just demonstrate how much care and love was put into the game even if only a few people would notice.

What the hell, that is amazing, I've never seen anyone that knew this, how were even some of those found? Star allies get too much hate, it is fantastic

You really took Dedede through all them rooms. He now knows every secret of the Galaxy

It's surprising how many secrets this game still holds I even feel like those rooms are even weirder than the HAL rooms just sublime

Fun fact: if you see a Broom Hatter just walking around in a place you wouldn’t expect it to be, you’re likely either in or near a secret area.

Edit: What? I wonder if I was onto something or just made that up

A big reason why these secret rooms are so obscure is that some of them weren't in the base game and were added in the post-launch updates. The secret rooms in Clash at Castle Dedede, Echo's Edge, and Planet Earthfall were added in version 3.0 alongside the Wave 2 Dream Friends, and the one in Planet Caverna's Miniboss rush was added in the final update.

By that point, most had already finished the story mode and wouldn't have thought to check the main story levels again for secrets. The only reason I knew about them was that the developers hinted at their existence in the Star Allies Switch News channel, which unlike a lot of other news channels on the Switch, shared a lot of fun tidbits that players may not notice, and they even shared some development details there.

It feels so weird that the Dedede and Midboss bonus rooms were so hidden, when they look prettier than the main stages they are built on. Wonder why they hid them so well

I’ve had my experiences with some of these hidden rooms and thought the devs put really hard work into designing areas for only the most adventurous of players. Prior to the video, I had found all of them except the Clash at Castle Dedede and Planet Caverna ones. I like the fact that they don’t contain anything mandatory for 100% so that you feel extra excited when you stumble across them.

I’m so impressed, I never knew there were secret rooms in levels I didn’t know about like the Dedede’s Castle level.

There's at least two secret rooms in this video that I didn't know they exist so, thank you.

Wth, I went as far as to do every single guest star mode with every single character (to give you an idea of how much I played this game), but never have I seen a single one of these rooms at any point (including the ending to the training room) this was an amazing video, and this just showcased just how much care the devs put into this incredible game. Thanks for making this video

Wow, I’ve clocked over 170 hours in this game and didn’t even know around half of these even existed. It really shows how much love they’ve put into this game, despite how it’s so disliked by the community (and OP’s determination to find them all!!)

They hid all these secret doors real good, I didn't even notice them at all.

Wow, I think the only ones I knew of were the lava one and the Planet Cavius Caverna one. The story mode may have been really short, but they put in so many secrets between these, the HAL rooms, and the dream friend portraits!

Finally someone knows that one sword move

this video is just kirby and dedede going on treasure hunting adventures across the galaxy

Level 1: "World of Peace: Dream Land"
Level 2: "World of Miracles: Planet Popstar"
Level 3: "Fortress of Shadows: Jambastion"
Level 4: "Far Flung Starlight Heroes"
Level 5: "Kirby Star Allies"

I'm only at the first of these and I already wonder HOW I never noticed that open window.
Having seen the entire thing, I must say that I only knew of the one in Nature's Navel (and the one at the very end, of course).

Really an amazing video! This is what true secrets in video games are. Sure, you get plenty of "secrets" to find nowadays, but in the end those count towards 100% completion in most games, so they're not really secret after all. They're known.
HAL has always been good with these true secrets, such as with the Moon in Dreamland 1's Bubbly Clouds, HAL rooms, or that UFO-room in Vegetabel Valley's first stage. The first two of these have become well-known over the years, so HAL has added them ON TOP of adding more true secrets like seen in this video.

Another game with a ton of true secrets like this is Super Mario Bros. 3, and it's one of the reasons why I call it my favourite 2D Mario game.

Ive played this game for years yet still didn’t know about half of these hidden areas, I should’ve kept my eye out for some of them!