I'm guessing the "Inverted metal thingy" you describe is the keep. The keep fits in the door frame. The bolt in the lock fits into the keep, when the key is turned to the locking position.
If you think that someone can easily force the lock then you should be assessing whether the door/frame is man enough to house the lock.
You can fit all the locks you want, but if someone comes prepared, there is no keeping them out.
The particular lock shown in the link would be fairly easy to open without a key... I would just hammer a chisel into the edge of the frame/architrave to create a gap in front of the lock. Then using a hacksaw blade just cut through the lock bolt .... Have done it many times. The only way to deter this form of entry is to fit a lock with hardened steel rollers in the bolt to resist sawing. Even then I can still get past them, but like I said .. "If you come prepared".
It isn't normal practice to fit a 5 lever sash lock with hardened rollers and a steel keep box on an internal door, although I have done it when creating a sanctuary (for witness protection). What do you actually want to do ?
https://www.safe.co.uk/products/union-2243-3-lever-sashlock-63mm.html?ACODE=googlebase&gclid=CPDGp9euhskCFUNAGwodCqIItQ