When you're having fun playing a game, you know it. And when you're playing a game that freaking sucks, you -should- know it. How do I make you feel the euphoria you get from playing a good game? That's the question we ask as game designers over and over again. Well, I don't have a specific answer, but I've been able to design some pretty fun games. And I know what fun games are like. So I'm going to design and create a game and allow you to get in my head as I do it. You'll see what I do, how I do it, and why I do it. Here we go.
The game

What do I want to make? Let's look at a sort-of-popular theme. PvP Espionage. So I have my idea for the game. Players will play as spies and use espionage to kill each other. Sounds fun, right? No. As a more experienced designer, I instantly point out a huge flaw: If two players are facing off against each other, they are more than likely to just skip the espionage part and dive head-first into killing each other. There is a solution to this used in Assassin's Creed: Brotherhood. Players could be pursued by one target as they are forced to hunt a separate one, kind of in a cycle. However, I want this game to be more invasive. So I settle on making it single-player, and forcing the player to take out AI with a specific goal in mind. Now, I instantly think, "How do I make this work in a way that forces the player to be stealthy?" Well, since the main theme of the game-play is stealth, that's where I'm going to have to be most strict. First, I think of ways to punish the player for failing to be stealthy. I'll decide that I'm going to make the AI strong enough to the kill the player quickly, and make them attack in flocks if the player loses his cover. This is all conceptual. So next, I think of how to reward the player for being stealthy. Well, in this game, your goal is to complete the mission by getting to the end and then escaping. I'll decide that having no harsh AI encounters is rewarding enough. So what mission shall I make? I'm going to decide that I'm going to have multiple different levels. Hmm, 10. Now each level will have some static pieces: A start respawn, a treasure, and escape tunnel.

Ok, so now I should decide what abilities I'll give the player. I'm going to go for simplicity. Left-clicking enemies will kill them. Right-clicking a body will allow you to disguise yourself as the character until you do something suspicious. Left-clicking a body will allow you to drag it until you left click it again, and will slow down your movement and block other conmmands. WASD will be used to move, since I'm focusing on mouse controls. Right-clicking enemies and other things will use an item you're holding. Items are dropped by dead bodies and can be found in the level, and range from bombs to keys. Left-clicking the ground will make you drop your item. Left-clicking yourself will make you toggle between running and walking. Okay, those are some whacky controls. We'll see how it works in the game. Now that we have the basic things a player can do, we must create objects to fill the level with that make the simple actions feel more dynamic.

First, let's think about the AI. I'll make them have specific walk-around areas(using areas). If they are killed while awake, they will make a loud noise that alerts nearby guards. When a guard is alerted, it will charge towards the player(if the player is in sight) and if it bumps into the player, the game restarts. After five seconds, if the player is not seen, the guard will return to an idle mode. If the guard sees a player, they will become alerted if they see a weapon or if the player is not in disguise. When a guard is alerted, it has a 20% chance to run away. If it gets away, it will alert guards in a large range. A guard can only see 5 tiles in front of itself. If a guard finds a dead body, it will become alerted. If you run, you will make a noise that alerts guards within 3 tiles. Noises and dead bodies will cause "inspection alerts" An inspection alert will make the guard wait three seconds before becoming alerted. If the player is wearing a guard outfit, then he/she will get away with holding an item. A player can sneak past a guard that is in inspection mode.

There are certain spots in the level where you can not be seen. These areas are marked with dark shadows.

There will be multiple doors in a level. Some doors may require keys which guards will be seen holding.

In some rooms, a light switch will be available for the player to use. Using it will shut off or turn on certain lights.

Stairs will take the player up and down floors in a level.

In some levels, buttons or levers may be needed to open doors or activate certain things.

There may be a power box in a level that turns off all lights in a building.

There are cameras that may see the player if they face it. They will alert nearby guards.

Not all AI are guards. Some may be normal civilians.

Some scenery can be pushed by the player by bumping into it

Some walls will be opaque, like windows or glass.

Glass walls can be broken.

Wood walls can be destroyed with an explosion.

Wood flooring can be destroyed with an explosion. This creates holes that drop to floors below.

There. Creative level design is absolutely key to creating a fun game. Simple game play, supported by very sturdy levels. Look at Mario, or Zelda, for example.

Now I make the game.

Okay, a couple hours later, I have a playable two-level demo. The level design is very intuitive, making for an excellent puzzle. However, since it is only the first two levels, it may be a bit difficult.
Click HERE to download the demo of Clandestine

If you're confused, here's a walkthrough.

First, walk down through the shadows hugging the wall over to the door. Wait for the green guard to look away, and then open the door and sneak through. The blue guy is harmless, so kill him with left-click and then left click the body and drag it into the shadows. Right click the body to disguise yourself from the guard outside, and then go out and approach the guard. He'll attack you if you step within one tile of him, so walk near him and wait for him to step towards you. Left click him when he does, and then drag the body into the shadows. Go back and get the key the other body dropped.(Holding an item ruins your disguise) Right click the locked door, and then drop the key by left clicking on the ground. Go up to the shadows, and kill the next guard in the darkness the same way you killed the first(Wait for him to step within 1 tile of you). Then drag his body to the edge of the shadows going into the next room, and leave it at the door entrance(this is important!!). Walk through the shadows to the light switch and right click it. The lights will turn on revealing the body, and the guards in the room will run towards it to check it out. Left click yourself to begin running and go AS FAST AS YOU CAN to the treasure, and then for your escape run to the glass wall and left click it to destroy it. Run back to the start, and you completed the level.

Welp that's all for today. I hope this was enjoyable =)
if u want i can help u. i am curntly planing two games one is being devlopd and another is being planed
I played through it, and I didn't really enjoy myself. ):
Maybe if you had an original gimmick/mechanic added to the classic invasion/ninja gameplay would make it a bit more enjoyable.
i talked about your game sorta in my video
This makes me want to make a game like this as well...
Megaupload? You know you have your own Byond hosting space, right =)