Strongpoint can vary a lot, from concrete position, to a well set basic dug in set of foxholes
Here is a few example:
Main defense line:
Wooded-Iron MG shelter
More organised, but still no concrete in there:
Use of Cellar:
To me a strongpoint is more a key-position you want to keep, where you put in your heavy weapons, that can support the other ''weaker'' element of your defensive system.
All those are germans BTW, it doesn't make much difference if the line in between is lightly held, because then your reinforce strongpoint will isolate the ennemy that has crossed the line, and a counter attack will whipe out the opposing force(that's german tactics). That's why 21st army group had a tactic of digging in when they were on objective and having a following unit overlapping them. It was to defeat armored counter attack on their newly acquired position.