Tunnelball: design a level for this flash game

// TUNNELBALL V0.1 BETA
// By Emanuele Feronato
// www.emanueleferonato.com
// visit my blog and find my tuotorials
// attaching movies
attachMovie("bg", "bg", 1, {_width:5700, _height:5700});
attachMovie("explosion", "explosion", 2, {_x:284, _y:209, _visible:false});
attachMovie("kira", "kira", 3, {_x:284, _y:209});
attachMovie("wall", "wall", 4, {_width:5700, _height:5700});
attachMovie("upper", "upper", 5, {_x:300, _alpha:80});
attachMovie("count_down", "count_down", 6, {_x:0, _y:0, _alpha:80});
attachMovie("legend", "legend", 7, {_x:0, _y:391, _alpha:80});
attachMovie("game_over_button", "game_over_button", 8);
attachMovie("splash", "splash", 9, {_x:440, _y:113, _alpha:80});
attachMovie("visit", "visit", 10, {_x:250, _y:433});
// the game_over_button properties aren't set in the attachmovie declaration as I did
// with the others movieclips because it didn't work. Seems that button's properties
// cannot be declared "on the fly".
game_over_button._visible = false;
game_over_button._x = 284;
game_over_button._y = 209;
game_over_button.onRelease = function() {
	getURL('http://www.emanueleferonato.com');
};
// attaching sounds
hit_sound = new Sound();
hit_sound.attachSound("sound_hit");
gameover_sound = new Sound();
gameover_sound.attachSound("gameover");
// player's starting positions
start_x = new Array(240, 240, 240, -2100, 2690, -1800, 1400, 2700, 200, 2700, 250);
start_y = new Array(0, 350, 0, 2500, 2800, 2500, 2000, 2600, 600, 2750, 190);
// level names
level_names = new Array("Intro", "Wide and linear", "Not so wide...", "Long but straight", "Crazy serpentine", "Use your brakes", "Intestine", "The bunker", "Back and forth", "Impossible: give up", "Congratz!!");
init_game();
// main function
kira.onEnterFrame = function() {
	if (kira_playing) {
		// timer 
		if (level>1) {
			elapsed_time = getTimer()-start_time;
		}
		time_left = countdown-elapsed_time;
		if (time_left<45000) {
			count_down.time_left.textColor = 0xffff00;
		}
		if (time_left<30000) {
			count_down.time_left.textColor = 0xff8000;
		}
		if (time_left<15000) {
			count_down.time_left.textColor = 0xff0000;
		}
		count_down.time_left.text = time_to_string(time_left);
		// detecting keys pressed
		if (Key.isDown(Key.LEFT)) {
			xspeed = xspeed-power;
		}
		if (Key.isDown(Key.RIGHT)) {
			xspeed = xspeed+power;
		}
		if (Key.isDown(Key.UP)) {
			yspeed = yspeed-power*upconstant;
		}
		if (Key.isDown(Key.DOWN)) {
			yspeed = yspeed+power*upconstant;
		}
		if ((Key.isDown(Key.SPACE)) and (brakes>0)) {
			yspeed = yspeed/2;
			xspeed = xspeed/2;
			// I don't want the player to lose brakes in the 1st level
			if (level>1) {
				brakes--;
				count_down.brake_left.textColor = 0x00ff00;
				if (brakes<75) {
					count_down.brake_left.textColor = 0xffff00;
				}
				if (brakes<50) {
					count_down.brake_left.textColor = 0xff8000;
				}
				if (brakes<25) {
					count_down.brake_left.textColor = 0xff0000;
				}
				count_down.brake_left.text = "Brakes: "+brakes;
			}
		}
		// speed calculation                                            
		xspeed = (xspeed+wind)*friction;
		yspeed = yspeed+gravity;
		if (xspeed>0) {
			this.kira.gotoAndStop(1);
		} else {
			this.kira.gotoAndStop(2);
		}
		// oh no!!! kira dies!!!
		if ((energy<1) or (countdown-elapsed_time<1)) {
			kira_die();
		}
		// screen update                               
		wall._y -= yspeed;
		wall._x -= xspeed;
		bg._y -= yspeed;
		bg._x -= xspeed;
		// collision
		if (wall.hitTest(this._x, this._y, true)) {
			if (!wall.end.hitTest(this._x, this._y, true)) {
				// if the level is not the 1st, the player lose energy
				if (level>1) {
					energy -= Math.round(Math.sqrt(xspeed*xspeed+yspeed*yspeed));
					count_down.energy_left.textColor = 0x00ff00;
					if (energy<75) {
						count_down.energy_left.textColor = 0xffff00;
					}
					if (energy<50) {
						count_down.energy_left.textColor = 0xff8000;
					}
					if (energy<25) {
						count_down.energy_left.textColor = 0xff0000;
					}
					count_down.energy_left.text = "Shield: "+energy;
				}
				xspeed = 0;
				yspeed = 0;
				hit_sound.start();
				wall._y = old_y;
				wall._x = old_x;
				bg._y = old_y;
				bg._x = old_x;
			} else {
				xspeed = 0;
				yspeed = 0;
				splash._visible = false;
				if (level>1) {
					energy += 100;
					brakes += 100;
					start_time += 60000;
				} else {
					start_time = getTimer();
				}
				level++;
				count_down.energy_left.text = "Shield: "+energy;
				count_down.brake_left.text = "Brakes: "+brakes;
				count_down.energy_left.textColor = 0x00ff00;
				count_down.time_left.textColor = 0x00ff00;
				count_down.brake_left.textColor = 0x00ff00;
				position_bg(level);
			}
		} else {
			old_x = wall._x+2*xspeed;
			old_y = wall._y+2*yspeed;
		}
	} else {
		// once the game is over 
		if (Key.isDown(Key.SPACE)) {
			init_game();
		}
	}
};
function time_to_string(time_to_convert) {
	elapsed_hours = Math.floor(time_to_convert/3600000);
	remaining = time_to_convert-(elapsed_hours*3600000);
	elapsed_minutes = Math.floor(remaining/60000);
	remaining = remaining-(elapsed_minutes*60000);
	elapsed_seconds = Math.floor(remaining/1000);
	remaining = remaining-(elapsed_seconds*1000);
	elapsed_fs = Math.floor(remaining/10);
	if (elapsed_hours<10) {
		hours = "0"+elapsed_hours.toString();
	} else {
		hours = elapsed_hours.toString();
	}
	if (elapsed_minutes<10) {
		minutes = "0"+elapsed_minutes.toString();
	} else {
		minutes = elapsed_minutes.toString();
	}
	if (elapsed_seconds<10) {
		seconds = "0"+elapsed_seconds.toString();
	} else {
		seconds = elapsed_seconds.toString();
	}
	if (elapsed_fs<10) {
		hundredths = "0"+elapsed_fs.toString();
	} else {
		hundredths = elapsed_fs.toString();
	}
	return "Time: "+minutes+":"+seconds+":"+hundredths;
}
function position_bg(level) {
	wall.gotoAndStop(level);
	count_down.level_name.text = "Level "+level+": "+level_names[level-1];
	wall._x = start_x[level-1];
	wall._y = start_y[level-1];
	bg._x = start_x[level-1];
	bg._y = start_y[level-1];
}
function kira_die() {
	gameover_sound.start();
	kira._visible = false;
	explosion._visible = true;
	explosion.gotoAndPlay(1);
	game_over_button._visible = true;
	xspeed = 0;
	yspeed = 0;
	kira_playing = 0;
}
function init_game() {
	level = 1;
	elapsed_time = 0;
	position_bg(level);
	yspeed = 0;
	xspeed = 0;
	wind = 0.00;
	power = 0.65;
	gravity = 0.1;
	upconstant = 0.75;
	friction = 0.99;
	energy = 100;
	brakes = 100;
	kira_playing = 1;
	start_time = getTimer();
	countdown = 60000;
	kira._visible = true;
	explosion._visible = false;
	game_over_button._visible = false;
	splash._visible = true;
	with (count_down.brake_left) {
		text = "Brakes: "+brakes;
		textColor = 0x00ff00;
	}
	with (count_down.energy_left) {
		text = "Shield: "+energy;
		textColor = 0x00ff00;
	}
	with (count_down.time_left) {
		textColor = 0x00ff00;
	}
}