Door with parallel sides in circular wall

[Anonymous]

I am trying to model a "medieval" tower in Archicad 11. But when I simply add a door to the circular wall, the sides of the door opening are not parallel to each other, but "vanish" towards the wall's center point. So, because the wall is very thick (around 3 metres), the door hole is significantly wider on the outside of the wall than on the inside.

How can I make a "rectangular" door in a circular wall? Thank you.

*Edit: It would be better to speak of a "tunnel" than a door.

[Andy Thomson]

I am still collecting content, and welcome any contribution so long as it is not redundant and/or inferior to what is there already. English parts are always good