I know, there are already 2 Threads.
http://empires.cw-hosting.net/viewtopic.php?t=881&postdays=0&postorder=asc&start=0
http://empires.cw-hosting.net/viewtopic.php?t=856&postdays=0&postorder=asc&start=0
Yet, it was unclear if it work and what to do.
So, here you'll finde some pictures together with an explanation.
The best thing first: It works ;)
1. Download Steam from http://www.steampowered.com.
2. Install Steam and create an account.
3. Use Steam to download Half Life 2 Demo.
4. Download the current version of Empires.
5. Put the Empires-folder into the HL2demo-directory.
It should look like this (well, I guess, you will have installed Steam to another path and your account won't be called cosmicblue2000).
http://img226.imageshack.us/img226/9426/step12yh.jpg
6. Launch Steam, login with your account (don't worry if your Steam doesn't look like mine, it's just a skin)
7. Find the Steam-icon on the taskbar. Normally at the right lower edge.
8. Rightclick it and choose "Games".
9. Rightclick Half-Life 2 Demo and select "Properties".
http://img159.imageshack.us/img159/7527/step21qu.jpg
10. On the new window, click "set launch options" on the general tab.
http://img93.imageshack.us/img93/3716/step35ge.jpg
11. Again a new windows. Put "-game Empires" into the parameter line.
http://img115.imageshack.us/img115/9919/step44dr.jpg
12. Close all windows and launch Half-Life 2 Demo from the game list.
http://img115.imageshack.us/img115/4397/step50nk.jpg
Don't worry, that won't bring up Half-Life 2 Demo any more. Instead you will see the Empires logo soon.
Congratulation, you can play Empires now.
But don't forget: You should read the manual first.
Or at least get yourself the necessary informations how to play here in the forums.
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Replace empires with whatever third party mod you want. What this also means is that you'll have infinite steam ids for any third party game. Not tried with any official game yet though.
ps. you will get some errors with some things,