First of all: I dont get it. The Titans are inside their town, so itīs all over. Or are there several towns they can flee to?
They leave? Perhaps they don't really have that high an energy need (or have freakishly low metabolism or a highly efficient metabolism), so they take a few bites and leave. If they're intelligent, perhaps they will themselves to do it.

I'm also leaning on the idea that they leave because Eren has to ultimately get back to his basement and unlock the basement for the goods his father left him. Anime convention tells me that his father should die and the key will be salvaged.

I'd say there are several towns to flee to. That would mean that no one has been breeched in 100 years, or no one bothers to communicate between cities anymore.

I also don't think that was an explosive trap. The mutant seemed to have arrived via a thunderbolt, which leads to me thinking:

1) It's sent by a god
2) It's extra-terrestrial in nature
3) Titans have science and they launched the mutant at the wall
4) The lightning was purely coincidental

So far the setting reminds me of Chrome Shelled Regios, while the gruesome man-eating nods to Claymore.