I can't think of any examples of flipping between icons/abstraction and particularity that work well in tower defense (comment if I've missed something). Also, abstract games tend to have little or no story line -- Vector is storyless, but Demons vs. Fairyland [ http://stormalligator.com/games/ ] is storyfull for sure).
Particularity works well for some TDs (Kingdom Rush!), while abstraction is better for others (Bubble Tanks: http://armorgames.com/play/4962/bubble-tanks-tower-defense ).
Vector TD is more abstract. It's a sci-fi / space TD, and the blips and bloops of light are bare hints at starships, laser towers, etc. Much is done with color and simple shapes. It lacks finely detailed drawings and animation, and therefore invites us to imagine more or to zoom out a few levels from the 'particular' somehow. It's fun, too.
Armor Games has a good 'abstract' about the Tower Defense genre: "Construct buildings of terrible power and menacing magic in preparation for an onslaught of monster hordes from taking your crystals, slaying cities or stealing babies. Tower defense games are some of the most popular games on Armor Games for good reason!" Though it makes you think that all tower defense is fantasy... but good for a glimpse of TD.Got me thinking about 'abstract' in another sense -- the defense games that aren't 'particular'. Bloons TD (http://www.coolmath-games.com/0-bloons-tower-defense-1) is more particular than Vector TD (http://onemorelevel.com/game/vector_td). In Bloons, monkeys pop balloons. You can identify darts and airplanes and supermonkeys -- the particularity makes it fun.