Ultimul is a tower defense game, but a little different from the norm. Most TD games either have fixed paths that enemies must follow (ala Immortal Defense) or an open area that you segment with your towers to force enemies to traverse a winding path, and be subject to fire (ala Desktop Tower Defense). In Ultimul, you're defending a space station against waves of attacking enemy ships, who move left-to-right, and path isn't really relevant.
Instead, it feels something more like a shmup, in the sense of waves of alien attackers, new enemy types showing up at times -- and with boss battles every dozen levels or so, something I hadn't seen before. Also, enemies can destroy your turrets, so it's not just a matter of losing when enemies slip past the last tower.
As with other TD games, this is not a fast-action title, but the game does allow you to select targets for your turrets. They'll shoot at the closest enemy by default, but in some circumstances that's not optimal.
New enemies, when they appear, have capabilities quite different from what's gone before, and you can be caught out having pursued an upgrade path that no longer works with your current opponents; in particular, investing in repair capability so your turrets can survive long enough for you to switch gears when things go wrong seems important.
Nothing hugely innovative here, but a few interesting tweaks on the genre, and a nicely polished freeware effort.