What is the meaning of "va'Ychap'u"?
Rashi: They concocted.
Radak citing Tanchuma Bechukosai 2: They 'covered' knowledge from the Creator. They said that Hashem does not see nor hear; "Azav Hashem Es ha'Aretz" (Yechezkel 8:12), "Mi Ro'enu u'Mi Yod'enu" (Yeshayah 29:15). The Aleph is in place of the Hei (the last letter of the root) - "Chipah Etz Beroshim [va'Ychapehu Zahav]" (Divrei ha'Yamim II, 3:5). They mean the same and have the same root.
Radak (28): Most Yisraelim feared Hashem, for they knew that He helps and harms. Many erred to serve Him via intermediaries, like Nochrim. The Torah forbids this! A minority denied that He helps and harms, and said "Mi Ro'enu u'Mi Yod'enu" (Yeshayah 29:15). Our verse teaches that they said so covertly.
Malbim: They said heresy about the Divine, e.g. ha'Meshanim (they say that there are two Powers), those who say that Hashem is physical, or deny Hashem's knowledge or Hashgachah, or slander Hashem with adjectives.
What is the meaning of "Asher Lo Chen"?
Radak: That is not good.
For whom were the Bamos?
Rashi: They were for idolatry.
Malbim: Initially they were to offer to Hashem - but this denies that there is one special place sanctified for Avodas Hashem (via Korbanos).
What is "mi'Migdal Notzerim"?
Rashi: They were towers for lookouts to guard (Yisrael put idolatry there).
Radak citing Targum Yonasan: From strong towers [to walled cities].
Why does it say "Ad Ir Mivtzar"?
Rashi: This is like "mi'Katan v'Ad Gadol" (Shmuel I, 30:2). Malbim - i.e. they made Bamos everywhere - in small places and big.
Radak citing Targum Yonasan: [From strong towers] to walled cities.
