Simplify the following text for a Grade 4 native English-speaking person.
Original: In this part , the sound is only 4 km wide , hence the strategic importance of maintaining a sea fortress at this location commanding one of the few outlets of the Baltic Sea .
Simplified: In this part , the sound is only 4 km wide . This is why it is so important to keep a fortress at this place commanding one of the few outlets of the Baltic Sea .