The devil and demonic forces have been stripped of any and all authority by Jesus Christ. Any place the enemy seeks to exercise influence is an affront against the authority of Christ and, in fact, illegal. The spiritual enemies of God and man have no right to exercise authority over humankind.
However, some humans are deceived and participate in the strategies of the enemy. The places where mankind is in agreement with Luciferian and satanic doctrines are marked by their control over mankind and the destruction that follows. A stronghold becomes established.
But a stronghold influenced by spiritual dark forces is not the presence and power of the lying spirit itself, it's mankind's mindset as a result of that influence.
To say it clearly, a stronghold is a mindset.