You can activate this effect even if there are no Ritual Spell Cards or "Gishki" monsters in your Deck.
You cannot activate this effect if there are 4 or fewer cards in your Deck.
While the effect of "Dimension Fortress Weapon" is applying, the effect of "Gishki Noellia" cannot be activated. Also, if the effect of "Gishki Noellia" is activated, but "Call of the Haunted" is chained, targeting and Special Summoning "Dimension Fortress Weapon", the effect of "Gishki Noellia" is not applied. (You do not excavate any cards from your Deck.)