徑到壇門 To the Altar Gate
大仙拽開步進前，三藏等隨後，徑到了壇門外。擡頭觀看，那裡有一座高臺，約有三丈多高。臺左右插著二十八宿旗號。大仙拽开步进前，三藏等随后，径到了坛门外。抬头观看，那里有一座高台，约有三丈多高。台左右插着二十八宿旗号。
逐句解 Line by line
The immortal strode out ahead, with Sanzang and the rest behind, and came straight to the outside of the altar gate.
拽開步: 拽 = zhuài, to stride out, pulling the legs along -- pypinyin reaches for zhuāi. 三藏: 藏 = zàng.
Raising their heads to look, they saw there a high platform, some three zhàng and more in height.
觀看: 觀 = guān, to look. 那裡有: 那 = nà, "there was." 三丈: the measure 丈 (zhàng) is kept as it stands rather than converted; one zhàng is about ten feet, so the platform is some thirty feet high -- tall enough to be seen from the tower.
To left and right of the platform were planted the banners of the twenty-eight lunar mansions.
插著: 著 = zhe, planted and standing. 二十八宿: 宿 = xiù here, a lunar lodge -- the twenty-eight segments the old astronomers cut the sky into along the moon's path, each with its own name and its own governing spirit. Flying all twenty-eight banners is a way of saying that the whole sky has been formally notified. 旗號 "standards, banners."
The novel stops the action to describe the altar, which is what it always does before a ceremony -- and the description runs for four passages, cataloguing every object before a word of the rite is spoken.
The twenty-eight banners announce the scale of the thing: this is not a local prayer but a formal approach to the entire sky, lodge by lodge. What accumulates over the next few passages -- censer, tablets, vats, stakes, scribes, effigies -- is an impression of serious professional machinery.
All of which Wukong is about to make irrelevant by talking to the gods directly. The inventory is being built up so that it can be knocked down.