Strictly speaking, there were two gate-like structures within the Under-way that led from Shelob's Lair to the lower levels of the Tower of Cirith Ungol. The first of these was a simple block of stone that held the passage within the Lair that led towards the Tower, preventing Shelob herself from using that tunnel. Beyond this block, the passage led on to the Tower itself, where a solid pair of brass doors guarded the way into the deep chambers beyond.
Our references on this matter are ambiguous, and none directly state whether the 'Undergate' was the block of stone or the doors opening into the Tower (or perhaps even both in combination). However, only one of these - the brass doorway at the end of the tunnel - is expressly described as being a 'gate', and on that basis we assume here that the Undergate was the gate into the Tower, and not the stone block that guarded the entrance to the Under-way.
(The relevant comments comes at the end of The Two Towers IV 10, The Choices of Master Samwise. After the Orcs pass in through the metal doors beneath the Tower, those doors are shut and bolted behind them, leaving Sam Gamgee alone in the passage. At that point we're told that 'The gate was shut', implying that this doorway was indeed the Undergate of the Tower of Cirith Ungol.)
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