You can use the API of this server to create your own client, it is not important which language you use, you just have to make sure you use the HTTP-API, which should be possible in nearly all programming languages.

With each request you have to provide the API-Version you want to use. Currently this is 1.0. You have to add this version information to each API call by adding /version/1.0 to the URL.
Currently the API returns only JSON results, with each call you have to tell the server which format you prefer. You can do this by adding /format/json to the URL.

Example Request from Javascript (used in the online client):

var jsonRequest = new Request.JSON({
    url: 'http://fourinaline.phpgangsta.de/game/getavailableenemies/version/1.0/format/json',
    onSuccess: function(responseJSON){
        $('enemyId').empty();
        Array.each(responseJSON.enemies, function(item) {
            var option = new Element('option', {value: item.enemyId, text: item.name});
            option.inject($('enemyId'));
        });
    }
}).send();

Here is the list of all available API calls you can use: