Good evening! It's always a pleasure to see some activity within the own Lothlórien board, and more so if that concerns the Lady of Light
Proposing additions to her concept has rarely been a very easy of a task; not as challenging as suggesting new abilities for Gandalf, but we're not really that far, I reckon. In your case, however, the idea appears to be quite interesting and smart. Not a totally new ability, but rather another possibility for her, tied to every citadel of the faction.
The lore basis behind the main premise is absolutely unexceptionable: Tolkien himself stated that none but the Dark Lord with the One Ring on his finger could have breached Lórien's defence. The very magic of Galadriel (combined with and enhanced by the special properties of Nenya) imbued anything growing and anyone dwelling inside her enchanted realm. She both shielded her domain from evil and even halted the passing of time within the borders of the Golden Wood, ending up with beautifying Lothlórien beyond measure, without any akin example in Middle-earth but the sole Rivendell. In doing so, Elves were able to defy the ineluctable destiny of decay of Arda and therefore flourish in their beloved woodland kingdom, where everything they were to craft was permitted to remain unaltered during centuries.
It's very brilliant of you to have recalled such themes. People often underestimate the actual powers of Galadriel or simply imagine her as the typical sorceress of tales. While it's certainly true that her abilities can go very direct and impactful, she's also an authoritative leader/strategist; no wonder why she was capable of keeping Sauron at bay and preserving the lands in her government from the encroaching shadows spreading all over the place! As you justly pointed out, it was her clairvoyance which allowed her to pierce Sauron's schemes and guess his true intentions, whereas he could not violate her impenetrable mind. What I really fancy of her wonderful character are exactly the many hues/sides of her personality and stature. A leader, an acclaimed Elven Queen, a legend for mortal folks and an extremely mighty sorceress by herself. All at once
Now, let us get back to the actual game: I don't think that your concept is bad or flawed by itself. The issue lies, instead, in the general context.
Getting on top of the citadel is a special feature that belongs to Saruman and will be made even more relevant in the next patch. On the other hand, the Edain Mod is widely known for its great efforts in the pursuing of uniqueness among all factions; simply said, every one of them must retain some special characteristics that others do not boast. Henceforth, too much similar systems don't sound very good under this light, lest they disrupt the mentioned pillars of the game.
Nevertheless, this doesn't obviously equal to saying that her design is doomed to not undergo any possible change whatsoever. If you take time to browse this thread, you will see that the heroine has been a major catalyst for a plethora of discussions, since the dawn of the English forum. Tons of posts and worthy proposals. The sole result of significant note was a comprehensive concept for Galadriel, widening her role and stressing on her formidable magical skills. You're more than welcome to leave your thoughts on the matter; it's called
Ancient Might.
Beside that, as you referred to siege and defence, you may be glad to know that the overhauled version of Lórien's spellbook has not been showcased yet. I can't tell you much, nor give away internal information, but one could anyway be hopeful enough to think that, in tune with the principles of the upcoming update, she might be given more chances to protect her people from foes and influence her realm in a more physical way, staying thus true to the books. Because, at the end of the day, if most of the factions need walls or barriers, the shield of the Golden Wood has always been represented by incredible prowess of the Mistress of Magic.