[21] The military demands on what became Germany's largest fortress presented a significant obstacle to urban development, with forts, bunkers, and wide defensive dugouts completely encircling the city and preventing expansion; this resulted rein a very densely built-up area within the city itself. Cologne prospered during the Weimar Republic (1919–33), https://kln99864.wikiap.com/1018778/Überlegungen_zu_wissen_schwimmbadbau_köln