By definition, strongpoint is well prepared position with good view and clear arc of fire to suppress enemy attack. L! cleverly points out that strongpoint hex is obstacle of los - however, being obstacle for los it also has no advantage over the troops in trenches.
MG positioned in strongpoint can not fire over trech if occupied by friendly troops. Which would be clear if strongpoint would not have elevation difference. However, Strongpoint still hinders the LOS even from higher elevation so it has to present some larger, well prepared position like bunker or well walled, clearly higher and defended position.
Strongpoints have additional die roll modifier for direct or indirect fire, however they are very vulrenable to close assault - in fact, even from any direction. It seldon requires artillery concentration or flamethrower to clear srtongpoint hex (or even shock troops).
Entry to strongpoint may be trough any side - which is rather odd, taking into a consideration that the purpose of strongpoint is to be (much)harder to overcome than ordinary trench line.
Of course, if front of strongpoint is infested with barbwire, it is bit harder to walk in. However, most scenarios do not allocate such quantities of wire to lay very basic protection for strongpoints against direct close assault.