Massive group of zombies that I knew for a fact that I didn't stand a chance against showed up outside of town, but thankfully milled around for a while before attacking. In an attempt to survive I moved everyone into the catacombs, dug into the mountain that protected the town's northern and eastern approach. I decided to be clever and use the artillery shells I had stockpiled to booby trap the (mostly wooden) town with incendiary traps. The traps were put together so no zombie would trigger one until they'd gotten well into town, but when it was set off the others would go as well and hopefully burn the lot of them to a crisp. After that was done we piled what supplies we could into the catacombs, set up a sandbag barricade in case they didn't all die, and waited.
At around 6:00 the next morning they started moving with purpose toward the town, most toward the catacombs and others to destroy random stuff. Then they reached the traps, as expected. As much as I hated to destroy my own town I was pretty excited to see my plan in action, so I was a little bit disappointed when the IED didn't manage to trigger the next one in line. It had started a grass fire and ignited the buildings on either side of the street, so there was some hope, but it likely wouldn't come fast enough to stop the horde from breaking down the door. Sure enough there were still about 15 singed but essentially unharmed zombies banging on the heavy steel doors. Six colonists fired into the advancing crowd, but I only had survival rifles and had to retreat after a couple of shots.
We separated into three groups of two thinking smaller groups would be more manageable, but I knew it was over. Two colonists were followed by the majority of the horde and cornered in a mausoleum without even getting a shot off. Two more colonists were overrun at the end of a long hallway but did manage to kill all but one of their four pursuers. The last pair holed up in an ancient cryptosleep room I'd unearthed earlier and actually managed to kill the three that had followed them initially, but one was too injured to fight afterward and they were overwhelmed when the rest came after them.
In hindsight, I made pretty much all the wrong choices for dealing with this. Still fun, 10/10 zombie mod.