"That's the old passage to Ravenholm. We don't go there anymore."
- Alyx Vance
The city of Ravenholm managed to remain under the Combine's radar for some time, but it was eventually discovered and shelled with Headcrabs, turning the population into zombies. The entrance to Ravenholm was sealed off and when Alyx gives you the tour of the Resistance headquarters, she (somewhat ominously) points out that they don't go there anymore.
And for about 20 more minutes, that's still true.
The Combine finds your location and attacks the headquarters, forcing everyone to evacuate. And of course, the only possible way out for you is the old Ravenholm entrance. You quickly find yourself in the midst of a ghost town, but the city is far from empty.
It's not uncommon for you to find yourself at the mercy of a horde of zombies (plus the occasional fast and poison types) and more than once I found myself backed against a wall. (Being the Achievement Hunter that I am, I made things harder on myself by only using the Gravity Gun throughout the whole Ravenholm experience) You will also occasionally hear someone speaking to you or yelling in the distance, sometimes reciting Biblical Scripture. This man turns out to be the local priest, Father Gregori, who has set up a variety of traps around the city (including falling cars, flying buzz saws, etc.) that can be used against the armies of the dead (or you, if you're not careful).
Ultimately you do escape from Ravenholm, but the experience (while short) still sticks with me today.