Main Unit

program Invader;
{
    Copyright (c) 2011 Steven Binns

    Licensed under the Apache License, Version 2.0 (the "License"); you may not
    use this file except in compliance with the License, as described at
    http://www.apache.org/licenses/ and http://www.pp4s.co.uk/licenses/
}
  {$apptype gui}
uses
  CRT, SysUtils, SDL, SDL_GFX, SDL_TTF,SDL_MIXER, enemies, Checks, globals;

const
  AUDIO_FREQUENCY : integer = 22050;
  AUDIO_FORMAT : word = AUDIO_S16;
  AUDIO_CHANNELS : integer = 2;
  AUDIO_CHUNKSIZE : integer = 4096;

var
  level, mousex, mousey : integer;
  timer : integer;
  scoretext : string;
  font : pointer;
  colour, colour2 : pSDL_COLOR;
  music : pMIX_MUSIC = nil;
  spawn : Boolean = false;
  destrect : pSDL_Rect;

begin
  //Initialise sound, video and assign some variables space and values
  SDL_INIT(SDL_INIT_VIDEO);
  SDL_INIT(SDL_INIT_AUDIO);

  if MIX_OPENAUDIO(AUDIO_FREQUENCY, AUDIO_FORMAT, AUDIO_CHANNELS, AUDIO_CHUNKSIZE) <> 0 then
    halt;
  screen := SDL_SETVIDEOMODE(400, 400, 32, SDL_SRCALPHA);
  fontface := SDL_SETVIDEOMODE(400, 400, 32, SDL_SRCALPHA);
  if screen = nil then
    halt;
  if TTF_INIT = -1 then
    halt;

  new(test_event);
  new(colour);
  new(colour2);
  tick := 0;
  score := 1;
  music := MIX_LOADMUS('John Stump - Death Waltz.mid');
  MIX_VOLUMEMUSIC(20);
  font := TTF_OPENFONT('C:\Windows\Fonts\Arial.ttf', 20);
  colour^.r := 0;
  colour^.g := 255;
  colour^.b := 0;
  colour2^.r := 0;
  colour2^.g := 0;
  colour2^.b := 0;
  level := 5;
  timer := (level * 300) + 5000;
  MIX_PLAYMUSIC(music, -1);
  //Begin the main loop.
  while stop = false do
    begin
      //Set the mousex and mousey variables to your mouse's x and y positions.
      mousex := test_event^.motion.x;
      mousey := test_event^.motion.y;
      scoretext := 'SCORE: ' + inttostr(score);
      if timer = (level * 300) + 4000 then
        spawn := true;
      fontface := TTF_RENDERTEXT_SOLID(font, PChar(@scoretext), colour^);
      SDL_BLITSURFACE(fontface2, nil, screen, nil);
      SDL_BLITSURFACE(fontface, nil, screen, nil); //draws the score in the top left of the screen
      if SDL_POLLEVENT(test_event) = 1 then
        begin
          if test_event^.type_ = SDL_KEYDOWN then
            begin
              if test_event^.key.keysym.sym = 27 then
                halt;                                //If esc key is pressed, stop the game
            end;
      end;
      if timer = 0 then
        begin
          level := level + 5;
          timer := (level * 300) + 5000; //after the timer runs out, advance the level
        end;
      //begin moving the enemies
      enemies.TICK_ENEMIES(mousex, mousey);
      //draw the character
      FILLEDCIRCLECOLOR(screen, mousex, mousey, level, $FF99006F);
      FILLEDCIRCLECOLOR(screen, mousex, mousey, level DIV 5, $0000FFFF);
      //begin drawing enemies
      enemies.DRAW_ENEMIES(screen, level);
      FILLEDCIRCLECOLOR(screen, mousex, mousey, level, $000000FF);
      if spawn = true then
        begin
          if random(100) < 10 then
            enemies.CREATE_ENEMY(level);
          //Check collisions between enemies and the character.
          Checks.CHECK_COLLISIONS(screen, level, mousex, mousey);
        end;
      inc(tick);
      if tick = 10 then
        begin
          inc(score);
          tick := 0;
        end;
      //Clear the score before the next frame.
      scoretext := 'SCORE:                                  ';
      fontface2 := TTF_RENDERTEXT_SHADED(font, PChar(@scoretext), colour2^, colour2^);
      timer := timer - 10;
    end;

  //Freeze enemies on the screen.
  for i := 0 to 20 do
    begin
      if mobs[i, 0] = 1 then
        begin
          FILLEDCIRCLECOLOR(screen, mobsx[i], mobsy[i], mobs[i, 3] DIV 2, $00FF00FF);
          if mobs[i, 4] = 0 then
            begin
              FILLEDCIRCLECOLOR(screen, mobsx[i], mobsy[i], mobs[i, 3], $FFFFFF6F);
            end
          else if mobs[i, 4] = 1 then begin
            FILLEDCIRCLECOLOR(screen, mobsx[i], mobsy[i], mobs[i, 3], $9600006F);
          end;
        end;
    end;
  //Create a destination rectangle for the final message.
  new(destrect);
  destrect^.x := 50;
  destrect^.y := 50;
  destrect^.h := 50;
  destrect^.w := 50;
  timer := 100;
  //It should fade the text in...
  //It didn't work...
  while timer > -255 do
    begin
      fontface := TTF_RENDERTEXT_BLENDED(font, 'You were assimilated...', colour^);
      SDL_BLITSURFACE(fontface, nil, screen, destrect);
      SDL_FLIP(screen);
      SDL_DELAY(10);
      timer := timer - 1;
    end;
  SDL_FREESURFACE(screen);
  SDL_QUIT;
end.

Remarks

Could you make such an effective game using SDL?