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. Her relics rest at the monastery of Santa Casilda.
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.