Creatures Caves

I don't know how much experience you have with other agents, but you might already know that the games don't support alpha channel. You can't have a partially transparent overlay with just a sprite file. The only thing you can do is making pixels fully invisible in the c16 or s16 files by setting them to a perfect #000000 black.

There are ways to use alpha with commands, but that'd sacrifice C3 compatibility and make it a DS-only agent. Also, the favplaces script might not like it if you futz around in it and may glitch out.

To make the room livable, you need to create emitter agents that put out Cellular Automata or CAs. In Creatures games, those are used for smell propagation. Heat is one of them. CAs are created by agents if you run the EMIT command on them.
So what you do is to make some invisible agents (I believe there should be something like a blank.c16 in every game), place them in strategic places around the room, and have them stench up the place.
Other CAs like nutrients and moisture are mainly required for plant growth and for creature navigation. On one hand, many plants don't grow unless there is a minimum of a certain smell, on the other hand norns plan their pathing by following the smell of the thing they want/need.

If you want your CAs to work right, you gotta LINK your rooms, and properly set their room type as well.

Lastly, there is no shame looking at references. Many modern agent compiling tools can DEcompile agents again. Pull apart some modern metarooms and see what they did. Just keep in mind that room linking is a relatively new discovery compared to how long third party agents have existed, so many of the older ones might be a little lackluster in that regard.