A fairly recent trend, unprotected building climbing, or 'Urban
Free Solo' as it has come to be known in the climbing community, is
more or less universally illegal, considered a form of trespass.
The most promonant name in the field is probably the French
climber, Alain Robert, who has climbed famous structures around the
world, and has been arrested for it several times.
Often, the style of climbing involved in this field is not
incredibly technical, but makes up for this by requiring a great
deal of stamina, and of course, the confidence and courage to
attempt such climbs without protection.