As an Army Veteran who has been to Iraq I am very familiar with the concept of military strongholds. In fact, while I was deployed I lived in a Forward Operating Base (FOB) which is essentially a modern day stronghold. And while the technology and tactics of war have changed significantly in the last few thousand years, strongholds still serve the same basic purpose.
A stronghold is simply a fortified location that the occupying military force can use to exert influence over the surrounding area. For instance, the FOB I was stationed at was surrounded by concrete walls and guard towers manned by Soldiers with automatic weapons and radios. This made it a relatively safe place for our troops to rest, get food, resupply, get medical attention and all kinds of other important things in between patrols.
Each day, these patrols would go out and exert influence on the area by seizing weapons caches, capturing terrorist leaders, and take out terrorist cells. They would then return to the FOB and get ready to do it all again the next day.