If you are at the very top of a big building then large windows decals will be seen for literally miles and this will be a fantastic way to get free advertising that would actually be more valuable than paying to have your logo printed on a billboard. The power of this form of advertising can then be increased exponentially if your advertising is included in photographs. This way any scenic photos people take of the area - of the high street for example, or of a view of the city at night, will include you logo or your advertising in the photographs. If they then upload these images to Facebook then they'll be seen by the thousands of people they likely have signed up as their friends and this will ensure that you reach a large range of people. Even better, there's a high chance that those people will be local - or that they will at least know the location. Again this kind of marketing will benefit on an unconscious level if nothing else.