Well to be fair to them, the fences had been doing a perfectly fine job for awhile of keeping the zombies out. And there was a double layer of them. So they probably didn't see any urgent need to construct something else.
I'm pretty sure they mentioned concerns a few times last season about walker build up at the fences so this isn't something that is taking them by surprise.
Also didn't episode one mention they have had this community for six months now and very little if anything has been done in that time to beef up the parameter around the prison.
We saw a small garden, a pig pen and some traps set in the woods but that is about it, as many people as they brought back that little bit of stuff could have been set up and started in a few days.