It's lots of fun on the surface and surprisingly deep beneath it, with plenty of value to boot.
Orcs Must Die starts very slowly, as you'll have only a limited number of traps at your disposal to stall and murder orc waves that try to make it to your magical rift. However, each level provides a new trap, combat spell, or enhancement that you can add to your arsenal. As the levels and enemy types become more varied, you'll be forced to try new strategies and make hard choices about what tools you'll equip for the job.
What really makes Orcs Must Die really work is how the different pieces fit together. Traps, spells, summoned guards, and your trusty crossbow are all pieces of the orc-slaying puzzle, and you'll need to use all of them to make it through the 8-hour campaign. Furthermore, you can upgrade your traps using skulls awarded after each level, which allows you to fine-tune your playing style and give you an edge where you need it. Levels see you setting up defenses, buying and selling various traps, and getting hands-on with the orc invaders using a small arsenal of weapons and spells that lend a good deal of action to this otherwise puzzle-esque genre.
The unlockable "Nightmare" difficulty is a surprise treat. I was expecting to simply face more monsters than before, but it also removes the grace periods that you normally would receive every few waves to restructure your defenses. As a result, Nightmare is pure chaos, lots of fun, and an entirely different experience, so it really adds a lot of value to the package.
It would be nice to see a greater variety of environments (lava-filled dungeon gets old rather quickly) and a melee weapon that isn't worthless, but these are small potshots at a great game. The lack of cooperative multiplayer does not go unnoticed, however, if only because the gameplay so obviously begs for it. Competitive multiplayer a la Toy Soldiers would not have been unwelcome either. However, this single-player adventure is definitely worth it and will not fail to satisfy your thirst for action, puzzling, and orc dismemberment.