Use the SecEditor. Also the tool is very sensitive and error prone. One wrong click is enough to fuck up the file. Also as you've noticed, it has a memory leak and becomes slower with every added sector.
How you should use it:
- Start placing all polygons first. Save the file.
- Add the first few sectors. Don't connect more than 3, maximum 4 polygons to a sector or you risk problems later on. Always do this clock-wise (if I remember correct haven't done this in a while). Save the file after every few sectors created.
- If you click a polygon twice, reload.
- At some point your *.sec file will be big enough to cause very low frames after just three or what added polygons. Save, restart the program, continue.
If you missclicked and didn't saved for a while, you can still fix this with saving the file now. Open the *.sec file and scroll to the bottom of it. There should be blocks like this:
4 0 0 0 0 0 0 384
If they only have 2 or 1 following lines, e.g. ...
4 0 0 0 0 0 0 384
... the block is broken and needs to be removed. Also at the top of the files, under the list of polygons, you get a number like "560" which is the amount of these blocks. They should match up as well. It saves you a lot time NOT to missclick, though. So just don't and save as often as possible. Also save under different names in case you accidentally fuck it up.
After you're done with creating the sectors, place a few random soldiers and let them walk around or use god mode and make them follow you, etc. always with the viewcone activated. This way you can see where fucked up sectors are located (pretty sure there will be a few at some point.. there are always some). If you know the position of fucked up sectors, delete them (and the closest polygons) in the SecEditor and rebuild them. That should do the trick.
It's a shitton of work, but if you got the workflow down, you'll be done quite fast. When working on the later maps of mine, it took me only about 30 mins or so to create a working sector file. Ofc it would be even faster if the tool wouldn't be such a pain in the arse.
The mission framework. You can only do what has been done in the vanilla missions. For example, if you want your specific commandos to enter a train which then starts a cutscene to end the mission... Well, sux to be you, that's not possible unless you also want boat or truck sounds at the same time.
Also in BCD you can script the telephone in, but not have the flashing light (and I think sound, but don't remember exactly) on the telephone pole.
Also the wrench and fence cutter from the sapper are not really usable.
Stuff like that is very limiting when it comes to designing new, unique missions.