[Viator]

I don't know if it will be of any use for anyone, but while gluing multiple open top castle towers I finally found perhaps the simplest and quickest method to glue the walking platform inside the parapeted top.

Option A.
This method (comprising of the set of gluing tabs around as the integral part of a platform) is often suggested by the authors of the models although it is perhaps the worst method to do the task. The platform often is to be glued inside the previously closed tube, so there is no means to secure the proper depth of the insertion and the internal sides of the parapet and of the battlement after the operation will be more or less dirty with glue.

Option B.
Far better method is to cut, bend and glue the four additional, separate gluing tabs to the internal surface of the walls of the tower before closing the shape of the building. This method secures clean and precise levelling of the platform, however requires precision of measuring distances and fixing of the tabs.

Option C.
Sometimes, when the superb precision is not necessary and a loadbearing connection is not required, we can speed up the process. The most simplified method I found is to cut a strip of thin cardboard as wide as high the platform shall be placed (establishing the length of the strip by eye), roll it loosely (seamless - not creating a pipe of the certain diameter!) and insert into the tower from below. The loosely rolled "paper spring" expands and touches the side walls. You can put some glue along the free end (as shown on the C1 sketch) to secure the position, level the bottom of the "rolled flat spring" with the bottom of the tower and then let the glue slightly dry. But you can as well not using any glue before but drip four droplets of the glue into the points of contact (from below or form above) after inserting the "rolled flat spring" - as shown on the image C2. Then you shall apply the glue along the upper edge of the roll and insert the platform. No gluing tabs are needed.

You can even make the roll slightly shorter and NOT glue the roll to the walls before gluing it to the platform. Then, after gluing the platform you can move it like a syringe plunger to get the precisely required depth and then glue the roll to the walls from below, applying glue to the four contact points and pressing for the moment.