I love how grim and serious everyone is knowing they are riding to almost certain doom and then you have Merry fooling around like a kid at the fairground riding a pony for the first time.
Cool music note-- the Gondor theme is usually played by the French horn, as we can hear in the main musical motive that begins when the beacon is lit before the mountain sequence starts. Especially when it's introduced in TT, however, the Rohan theme is played with violins (think the Riders of Rohan track). At 3:18, it's the French horns playing the Rohan theme, creating a sense of unity between the peoples subcinsciously before we begin seeing the people of Rohan muster to fight and aid Gondor.
"Now is the hour, riders or Rohan! Oaths you have taken! Now, fulfill them all! To lord and land!" One of the most underrated line deliveries in the entire trilogy.
Always chills the spine