I started my X-COM adventure decades ago with TFTD.
You started with a good one then. I know Enemy Unknown/UFO Defense is the more commonly liked game but TFTD was much more intense in my opinion.
...I decided to wait 20+ turns to lure them...
That probably did it.
The reason:
(If I'm not mistaken)Spoiler:
After turn 20+ (I'm not sure of the exact number) aliens become more aggressive in hunting X-Com soldiers, this behavior change may have allowed them to enter the sub.
I tried to find an article about this on the
UFOpaedia site but could not locate the information so I could be in error.
I only mentioned the original Skyranger as an example of enemy AI units entering the map block with an X-Com craft on it. As you've said the subs having doors (In theory) blocking enemy line of sight should change everything as they have no way to detect X-Com's Aquanauts.
Are you using the
OpenXcom TFTD Patch?
(If you used the Windows installer it should already be applied)My understanding is that patch fixes a few errors in the map data. It is possible there was a microscopic hole in the Sub's map data that an alien glimpsed one of your soldiers through before entering the sub to attack.
I'm not sure about the exact map fixes delivered by the
OpenXcom TFTD Patch so this is just a guess.
From what I've read the AI in openXcom is
slightly different vs the original release. So if you are very familiar with the original the enemy units may sometimes do something unexpected.
I haven't done any map editing yet but from what I have read so far I don't think door tiles can be assigned with an attribute prohibiting enemy units from opening them.
It could be that you just got very lucky vs the Tentaculats in the DOS version.
If you really want to prevent the aliens from entering an X-Com sub and don't mind doing a bit of modding: I think that
OXCE allows for the creation of a map tile the aliens won't step on... but I may be mistaking it for the "
Death Trap" tile.