public void improveCity() {
        if ( selected.getClass() == Worker.class ) {
            Tile tile = selected.getTile();

            if ( tile.getCity() != null ) {

                ((Unit)selected).owner.removeUnit( (Unit)selected );
                tile.getCity().Production += IMPROVE_CITY_PRODUCTION;
                tile.getCity().PP += IMPROVE_CITY_PRODUCTION;
                selected = tile.getCity();
            }
        }
    }