We're trying to introduce Lucena, a 1 year old shy Siamese female kitten, to Elsa, a 3 year old extrovert Siamese female cat, living with us in our apartment since two years. Both are spayed.
We noticed Elsa tends to be very aggressive with new cats, so we're trying to introduce Lucena slowly. At the moment Lucena stays in a study room behind closed door, to keep her out from Elsa. Once-twice a day we let Lucena out to explore the rest of the house while locking Elsa in the study, so she can know Lucena's smell.
Three weeks went by, and one particular behavior by Elsa seems to be constant: she spends lots of time, sometimes an hour, focused and looking at the bottom gap of the closed door, with directed ears and the tail sweeping left and right against the floor. During that time she seems mostly unaffected by our calls, toys, laser or food. When we enter the room, she always tries to enter as well. And after Lucena has her time outisde and Elsa is back, she looks suspicious and walks around the same spots Lucena walked before. It's like she's tracking her smell.
On the few occasions they physically met by mistake (five times so far, last time yesterday), Elsa attacked Lucena with no second thought. Fur and spit were on the floor, and we had to separate them immediately.
A tip I read often, is to let the cats eat at opposite sides of the door to let the other cat's smell be associated with positive things. We started doing that a few days ago, and after the first few meals Elsa does indeed eat almost continuously (except stopping for a few seconds while hearing crunching sounds from the other side of the door), but few minutes later she's back at guarding/patrolling the door.
Is this behavior normal for a cat getting to know a newcomer? Or is it something we should be worried about? Any actions we should take before proceeding to let them eat with visual, non physical contact?