AS2 OOP: Scope and dinamicly created clips

programming flash
1

I was reading tonypa’s tile based game tutorial and tried to translate it to as2 when I realized two things:
a. I don’t know/remember what the this[stuff]._property syntax means, I don’t recall seeing [] except in arrays. can someone explain/link to an explanation/name to so I’ll know what to look up.
b. I don’t know how to scoop my individual tiles. my code is the following:

 class game{
    var _tileW:Number;
    var _tileH:Number;
    var _mapColumns:Number;
    var _mapRows:Number;
    function game(tileH:Number, tileW:Number){
        this._tileH = tileH;
        this._tileW = tileW;
    }
    private function buildMap(map:Array){
        var i;
        var j;
        var tag;
        _root.createEmptyMovieClip("container", _root.getNextHighestDepth());
        this._mapRows = map.length;
        this._mapColumns = map[0].length;
        for(i=0; i<_mapRows; i++){
            for(j=0; j<_mapColumns; j++){
                tag = "t_"+i+"_"+j;
                _root.container.attachMovie("tile", tag, _root.getNextHighestDepth());
                _root.container.tag._x = _tileW*j;
                _root.container.tag._y = _tileH*i;
                trace(_root.container[tag]._x);
            }
        }
    }
}

obviously _root.container.tag. isn’t the way to get to the tile, so what is?

thanks in advance =)