schwer zu erschütternde Bastion

[edmont]

I wonder if I have correctly translated the red phrase into English? Thanks very much for any assistance.
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Bis dahin hatte das Kartell der Produktionsmittelbesitzer, verfilzt mit den politischen Hilfstruppen und Agitationsvereinen des Herrschaftssystems, eine schwer zu erschütternde Bastion gewonnen.

Until then, the cartel of the owners of the means of production, entangled with the political auxiliary troops and the ruling system’s agitation groups, had gained a bastion that was difficult to shake.

[Keswick]

Absolutely spot on, Edmont! 

[edmont]

Thanks very much Keswick!