This module is awesome. Like the other Core 1-X modules it requires an experienced DM and party to make it a success, but with some good planning you can give your players a truly unique experience. With the horde advance mechanic you can get a nice bit of tension growing throughout the session and with so many options for players you get some good replay value.

If your players aren't splitting up, I would recommend nudging your players into investigating the orcs first, because it has some very interesting encounters and scenarios. This way, if the players mess up the mission and don't get to experience the portal missions they still did the best part of this.

It is not necessary to play the first two parts of the trilogy, but playing though them definitely gives some background that allows your players to feel more connected to the town and have some honest emotional reactions if they are not able to save it.

The ending can be a downer for players who like to "win" every adventure, as there is a very high probability that parts (or all) of the town end up in ruins. I didn't have this problem, but I have heard many express this concern with the adventure. If you play with adults, it should not be a problem, as everyone knows you can't win them all!