Casilda is from the Arabic al-Qāsim — but Saint Casilda of Toledo (11th c.) was the Moorish princess, daughter of an Islamic king of Toledo, who secretly brought food to Christian prisoners; converted to Christianity and lived as a hermit.
Patron saint of Burgos.
Casilda does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Casilda reduce to 4, The Builder. This is a traditional interpretive system, not a factual claim about the name.