suru verb (irregular), intransitive verb
Used for people gathering casually, often in a public place, sometimes with a nuance of loitering or hanging around without a clear purpose.
駅前に若者がたむろしている。
Young people are hanging out in front of the station.
公園に猫がたむろしていた。
Cats were gathered in the park.
suru verb (irregular), intransitive verb
assemble (troops); be stationed; be quartered
Military or historical use: troops gathering or being stationed. Rare in modern everyday language.
城の前に兵がたむろしていた。
Soldiers were assembled in front of the castle.