You get rythm when you repeat sounds in a pattern. If you make one soft tap on your desktop, you ain’t got rythm. If you make a strong tap on your desktop, you ain’t got rythm either. If you make a succession of aleatory soft and strong taps, likewise. If you make a pattern, like tap-tap-TAP and repeat it over time you’ve got rythm:
tap-tap-TAP — tap-tap-TAP — tap-tap-TAP …
or
tap-TAP —tap-TAP — tap-TAP …
or
TAP-tap-tap — TAP-tap-tap — TAP-tap-tap …
In poetry, those patterns are called metrical feet. In English poetry, the soft taps are unstressed syllables, and the strong taps are stressed syllables. In Classical Greek poetry, the soft taps are short syllables, and the strong taps are long syllables. (I beg forgiveness from our erudites for my simplism.)
(A few English poets have used the short/long pattern, but let’s not get into that.)
“Lepanto’s” meter is tap-tap-TAP (unstressed-unstressed-STRESSED). That foot is called an anapest. There’s four anapests on each line, so “Lepanto” is written in anapestic tetrameters (tetra means four in Greek, and a tetrameter is a line consisting of four feet).
Greek poetry written in anapestic tetrameters would consist of lines with 4 feet each, and each foot would be short-short-long. (By long and short we mean the length in time of their vowels: pit would be a short syllable, and Pete a long one.)
Note that poets like to add some spice to their lines, so not all of Chesterton’s feet are anapests. ‘White founts’ is a spondee (STRESSED-STRESSED),and ‘falling’ is a trochee (STRESSED-unstressed), so his first line’s four feet are (spondee)(trochee)(anapest)(anapest):
(WHITE-FOUNTS) (FAL-ling) (in-the-COURTS) (of-the-SUN)
I notice that many of “Lepanto’s” feet have an extra unstressed syllable, so they sound tap-tap-tap-TAP. If you read just one line with many variations out of context, the anapestic rythm is not very apparent, but when you read several lines together the rythm jumps out.