This one ... is one of the best tower defence game I ever play. I was looking for this game a long time to came to STEAM from independence studio. I was playing this game a long time ago as an flash version ... and even that time this game was one of the best.
Game is great mix of tower defence game and hero RPG. You can XP your hero inside battlefield up to level 10 and also each hero got their own special abilities. There is a system of buildings upgrades (like better sniper shots, or poison arrows, and other stuff) and spell upgrade (battle spells ... like meteor or patriots).
Game difficulty (hard as usual) is pretty fine and sometime can be really hard even in base game system. Game also provides to "hard core" players a few more difficulty levels on each one of base levels as it is ... HEROIC ... game difficulty on each level provides to player some more "restrictions" on each of battlefields (for example you cannot use hero, or use only archer buildings, ...). And of course there is another "BLOOD HARD CORE" level on each battlefield ... and this one is called an ... IRON ... game difficulty level, that restricted you abilities even more.
Game is pretty hard so its better to find somethin easier for the kids and let adults beat this one for them.
Game is also a time eater ... more then week of gaming to beat this to be sure (I mean HARD settings and also HEROIC and IRON stages).
Worth any money (best in sale) ... one of the best tower defence games I ever play.