Most notably were the levels that moved during gameplay, forcing you to quickly adapt your strategy for the new enemy paths and tower tiles. (playstationlifestyle.net)
See, most tower defense games handle enemy paths in one of two ways: either with fixed paths that you have to place your towers around (like the Sentinel games) or by using the towers themselves to create your own path (like the Fieldrunners games). (toucharcade.com)
-- Smart enemies — there are no predefined enemy paths. (wccftech.com)