What is Aliyas ha'Mekerah?
Rashi, from Targum Yonasan: It was the upper story of a summer house, for air and to cool off 1 . Radak - it had many windows.
Radak: Mekerah is a noun, like Machtah. There would be a Dagesh (dot) in the third letter, if not that it is a Reish (which does not receive a Dagesh). It is based on Kar (cold).
Why did he say "I have Hashem's word for you"?
Rashi: You must stand to hear it.
Did Eglon get reward for standing to hear Hashem's word?
Rashi (from Tanchuma Vayechi 14): Due to this, he merited that Rus come from him. Radak - she was his daughter (Sanhedrin 105b 1 ), and the mother of kingship (Bava Basra 91b).