What is the meaning of "Nikar"?
Rashi: He shut in [for me]. This is like "v'Al Tere v'Yom Achicha b'Yom Nachro" (Ovadyah 1:12). This is a strong conjugation (Pi'el), like "Ibed v'Shiber Bericheha" (Eichah 2:9). Had the verse wanted to use a soft conjugation (Kal), it would have said Nachar.
Malbim: He handed over [to me]. There is no similar usage in Tanach. It seems that [the verse says so] because it is like Nechar. I can act (fight) with him like [with] a Nochri, an enemy of Yisrael who came to a fortified city as a rebel. I.e. he came to prepare war, to make fortified cities his stronghold.
Why did Sha'ul think that the doors and bolt [of the wall] would help to ensnare David?
Rashi: David will trust in it, and not be concerned to flee from the city.
Radak: David will not be able to leave. Sha'ul did not think that anyone will tell David.
