"In the Round Tower at Jhansi, June 8, 1857" starts in medias res—that is, right in the thick of the action. It drops the reader into a chaotic, frightening situation. Understanding this situation requires understanding the historical context, starting with the reference in the title.
On June 8, 1857, a massacre occurred at the Jhansi fort in India. The Sepoy Rebellion or "Indian Mutiny" was underway, as Indian sepoys (soldiers recruited to work for the British) rose up against the harsh rule of the British East India Company. Captain Skene, a British officer and superintendent of the area, ordered the Europeans under his charge to take refuge in the fort. He believed a deal had been struck to prevent the rebels from killing the Europeans. As the poem's first stanza suggests, he was wrong.
Right away, the speaker depicts the situation as hopeless. Line 1 emphasizes, via hyperbole, that Skene and his wife are totally outnumbered: "A hundred, a thousand to one." In real life, the Skene family fell to a much smaller band of rebels (a group from the 12th Bengal Native Infantry). By exaggerating the numbers, the poem suggests how scary and overwhelming these rebels seemed to Skene and his wife.
The first line also contains multiple caesuras:
A hundred, a thousand to one: even so;
These pauses break up the poem's rhythm before it can get going, making the line sound stuck—as if, like Skene and his wife, it has nowhere to run.
Line 2 is straightforward enough: the couple has no "hope in the world" of surviving. They know they'll die one way or another, but they have a chance to consider how that death will happen.
Lines 3-4 describe the approach of the Indian rebels as they break into the fort. The poem takes a one-sided, conventionally Victorian perspective: the Indians are depicted as evil, the Europeans as strong and honorable. The speaker describes the rebels as "swarming howling wretches," more animal than human (insects swarm, wolves howl). Though biased and disparaging, this description captures the fear the rebels inspire in Skene and his wife. Line 4 uses repetition to heighten the sense of terror, narrating how the rebels
Gained and gained and gained.
This intense diacope illustrates how the rebels draw closer and closer, bringing the certainty of death with them. Notice, too, how the repetition seems to slow time down: the rebels "below" keep "gain[ing]" on the tower, but they haven't reached it yet. This gives the poem a chance, in the following stanzas, to zoom in on Skene and his wife as they confront their final moments. (Of course, the poem hasn't fully explained the context of their deaths, but as it was written shortly after the massacre at Jhansi, UK readers at the time would have been familiar with the subject matter.)