My favorite tower defense game of all time. It has literally everything you want in a TD.
* Strategic depth - the ability to actually make choices that matter and having to rethink if your choices didn't work.
* Increasing difficulty levels.
* Extra challenges (heroic mode, iron mode).
* Great music & sound effects.
* Addictive gameplay.
* Secrets - achievements - clickables.
The game is available (not with all the levels, and not with the element of heroes) for free as a flash game. Some people may consider this a negative (amagad paying $10 for a game I can get for free?) but I consider it a positive - the free version is easily accessible, treat it as a demo or play it whilst waiting for this version to go on sale.
Kingdom Rush on steam is 100% worth the cost for the amount of entertainment it will offer.