A stronghold is a fortified place, a place of security or survival. It is a place dominated by a particular group or marked by a particular characteristic.
Stronghold can not be easily accessed without first binding the strongman dominating the place we can see this in Mark 3:27.
As Christians, we should not give room to any demonic strongholds wherever we find ourselves.
We should be ready to face any demonic stronghold because we have a strong weapon that can pull down any stronghold whatsoever just like what Apostle Paul said to the church in Corinth in 2 Corinthians 10:4.