The Good: Charming art style, lots of traps, plenty of maps, funny one-liners...
The Bad: ...that grate on your nerves after a while, no multiplayer, difficulty spikes, orcs units get boring after a while, maps start looking the same, takes too long to earn upgrade points
I am not a real big fan of tower defense games because they become so repetitive too quickly, and are flooding the mobile phone market. OMD is a different take on the genre where you actually control the person laying the traps to stop the orcs, so the game actually involves the player into the action instead of you just sitting back and watching. There really isn't much of a story, but that's OK because the game is full of awesome gameplay, but after a while you will wish you could bring a buddy along, but alas, there is no multiplayer.
You start out by laying traps around the map, but you get to survey it first before the orcs come in. This is key to know the choke points and weak spots of each map. You start out with just a few traps like spikes, wall crushers, and lightning zappers, but as you beat each level you earn new traps and you can upgrade them to do more damage, more AoE damage, and even size. These traps are awesome because they are all part of the whimsical art style and tongue-in-cheek humor the game provides. Once you lay out your traps you watch the orcs come flooding in from different areas, but you have a crossbow and magic powers to help keep them at bay. I stood back behind choke points and just let loose with my crossbow button held down making sure orcs didn't get through. Once I got the trap that let me put archers and knights in different areas this helped a lot. However, as the game goes on it gets really difficult and this is where you will wish you could bring a buddy along.
The maps after a while start looking the same and this is where the game will test your nerves. Will you want to complete all the maps? The best thing to do is to have a friend take over and maybe play every other map after a while. Some maps were so hard, even using multiple strategies I still failed from protecting my portal and letting too many orcs through. This is where co-op would have made some maps a synch. The maps won't just get on your nerves, but the hero's one liners will which seem funny at first, but after a while you will wish you could turn that crossbow on him. There is some DLC available like costumes and a few more maps, but I really wanted more interesting enemies to kill. I started seeing green in my sleep because I saw so many orcs. Some are big and slow, some are fast, some have weapons of their own, but in the end the game kills you on repetition. I would have liked to see some mini-games or maybe some sort of reward/achievement system.
When all is said and done, Orcs Must Die! is a fun game in short bursts, and it really needs co-op to make the game a little easier and more enjoyable. The snappy one-liners grate on you after a while, and the maps start looking the same just shifted around. Even the orc units get boring to kill after so many maps. I played about 20 myself and couldn't take it anymore. If it wasn't for the difficulty spikes, it was the sheer monotony of everything. Once you unlock all the traps there's nothing to really look forward to anymore, and it takes forever to earn upgrade points. This is a fun game, but only for so long.