Milon's Secret Castle

Milon's Secret Castle is a sidescrolling game released on the Nintendo Entertainment System in 1986 in Japan and 1988 in North America, and developed by Hudson Soft. It's hard. Really hard.

Your Player Character, the titular Milon, is dressed in a very pixellated representation of blue overalls (or perhaps pajamas) and a blue hat. Your mission is to rescue Queen Eliza, who is held captive by the Evil Warlord Maharito, by struggling through several rooms filled with secrets and enemies. Your means of defense (or offense, as the case may be, although at times you may feel very much more hunted than hunter) is an infinite supply of bubbles. One hit is enough to take down most enemies, although the majority of them require concentration to be hit. Health capacity is small, but Milon can expand it by collecting items resembling honeycombs.

It received a Super Famicom sequel of sorts in Doremi Fantasy, which exchanges the block puzzles for platforming and is only really related in that it also happens to star Milon, who still has his trademark Bubble Gun.

Tropes used in Milon's Secret Castle include:

Bonus Stage: By touching certain boxes, Milon is transported to an area in which he collects music notes to earn money.
Bubble Gun: Milon shoots bubbles to defeat his foes. It works.
Check Point Starvation: Subverted. You can continue if you know the code. Otherwise, you're screwed. Thankfully averted in the Game Boy port, which gives you a password on a Game Over.
Classic Cheat Code: Want to continue your game when you get a Game Over? You'll need to know a special button input! (It's hold LEFT on D-Pad when you push START)
Fake Difficulty: Infinitely Respawning Enemies, no Mercy Invincibility, and completely unintuitive roadblocks.
Game Music
Game Over: And over and over and over.
Guide Dang It: To continue in the game, you must push certain blocks aside. There is no "pushing" animation or change when you move up against a block. Nowhere in the game or manual does it say that this can be done. The one block you must push looks exactly like all the others in the game. The Angry Video Game Nerd was not amused.
Heart Container: Mercifully present, although rare and difficult to find.
Hit Points
In-Universe Game Clock: Only outside. Stay out too long and nighttime will slam into effect, along a torrential downpour of lightning flakes. It reverts back to day if you can survive the lightning.
Jump Physics: Vital to success. Buying different kinds of shoes stacks up progressively more unrealistic and useful jumping abilities.
Money for Nothing: Money is vitally important to progressing, but one particular room, unlike others, regenerates its money.
Nintendo Hard: Just go look at the description for Nintendo Hard and you've basically got a summation of this game.
Respawning Enemies: The common enemies respawn after just a few seconds. The bosses never do, which is nice; some of their lairs are vital access points to other parts of the castle.
Save the Princess: The point of the game.