Daniel 6:11דניאל ו׳:י״א
When Daniel learned that it had been put in writing, he went to his house, in whose upper chamber he had had windows made facing Jerusalem, and three times a day he knelt down, prayed, and made confession to his God, as he had always done.
Berakhot 31a:19ברכות ל״א א:י״ט
Many halakhot are derived from evoking the prayers of biblical characters. Rabbi Ḥiyya bar Abba said: One should always pray in a house with windows, as it is stated regarding Daniel: “And when Daniel knew that the writing was signed, he went to his house. In his attic there were open windows facing Jerusalem, and three times a day he knelt upon his knees and prayed and gave thanks before his God, just as he had done before” (Daniel 6:11).
Berakhot 34b:29ברכות ל״ד ב:כ״ט
And on the topic of prayer, Rabbi Ḥiyya bar Abba said that Rabbi Yoḥanan said: One may only pray in a house with windows, as then he can see the heavens and focus his heart, as it is stated with regard to Daniel’s prayer: “In his attic there were open windows facing Jerusalem” (Daniel 6:11).
Shulchan Arukh, Orach Chayim 90:4שולחן ערוך, אורח חיים צ׳:ד׳
It is necessary to open doorways or windows facing Jerusalem, so as to pray opposite them. And it is preferable for a synagogue to have twelve windows.