Strange...
I did some research for you, in old German language, German poets language, military language, sports language and English/German translations for troop and Turm - and there is absolutely no reference to "a troop of horsemen" connected with "Turm".
You may find expressions like "the impressive horseman on his huge black horse was standing tall as a tower in front of me..." - but I have no idea, how Collins comes up with their explanation.
I dare to claim they are wrong.
There may be some misinterpretation or bad link in the data base.
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Ah, I guess I can explain it:
"Ein Reitersturm" would be a troop of horsemen storming something, maybe a hill.
The English expert for Collins' German translations has got something wrong here.
Reiter = horseman/men (no difference for plural in German)
Sturm = storm
Reiters may be a misinterpreted plural, or the genitive construction "des Reiters Turm" - wich means 'the horseman's tower' - his tower.
ReiterSturm is not ReitersTurm!
Damn am I good!
I'd like to vote, if I want the British still with us in the EU. Would be only fair.