the State Department was monitoring European progress in Monster technology. No-one wanted to fall behind or allow a Monster Gap to open.
The architects have cleverly designed the Monster Tower to look like a Corymorpha nutans hydroid so that no-one will notice it.
The ring of portals at each of the three levels contain powerful hydraulics that can open the great steel doors in seconds and spill monsters out across Paris.
Also if we renege on our side of the deal, the Old Gods will stop providing us with Marmite. That probably explains the involvement in the Monster Tower of an Australian religious subsidiary / breakfast-food manufacturer.