development
There was nothing to prevent other coastal fortifications from being developed, nor to prevent research of any sort. It would have been perfectly legal to build 18" (or even bigger) guns for army use, or even for naval research purposes. It would only be a violation if construction of a fort in the designated area, or a ship that would violate the treaty, that would be a problem. However, there are also important provisions in the treaty, specifically Article XXI, to account for unexpected national security issues. Finding out that a nation is stockpiling materials for a crash building program would certainly count. (The longest lead times for capital ships were for guns and reduction gears--if those are stockpiled, along with armor, building the ship is suddenly much faster.)
Article XXI
If during the term of the present Treaty the requirements of the national security of any Contracting Power in respect of naval defence are, in the opinion of that Power, materially affected by any change of circumstances, the Contracting Powers will, at the request of such Power, meet in conference with a view to the reconsideration of the provisions of the Treaty and its amendment by mutual agreement.
In view of possible technical and scientific developments, the United States, after consultation with the other Contracting Powers, shall arrange for a conference of all the Contracting Powers which shall convene as soon as possible after the expiration of eight years from the coming into force of the present Treaty to consider what changes, if any, in the Treaty may be necessary to meet such developments.