Printed signage next to the turnstile explains the €3 top-floor supplement and displays a large QR code. The visitor scans it with their phone camera — no app download required. Key design principles for the sign:

• QR code minimum 10 × 10 cm — tourist lighting is often dim
• Mount at eye level (150–160 cm from floor)
• Payment logos (Visa, Mastercard, Apple Pay) displayed for trust
• Short URL behind the QR — e.g. mostadome.mt/top
• Available in English, Italian, German and French

A mobile-optimised, Mosta Rotunda-branded payment page loads instantly in the visitor's browser. Stripe handles all card processing — the church never handles raw card data. The full payment takes under 30 seconds.

• No login, no account — ever. A tourist will not create an account for €3
• Apple Pay / Google Pay shown first — single Face ID tap, under 5 seconds
• Standard card entry as fallback (Visa, Mastercard, Amex)
• Page detects browser language — can serve in EN, IT, DE, FR
• Stripe's existing account can be reused from the main ticketing system

Stripe fires a webhook the instant payment succeeds. The backend validates it, creates a one-use token, and sends an HTTP command to the on-site relay controller — which pulses the Gunnebo turnstile's dry-contact input, opening the gate for exactly one passage.

• The turnstile receives the same relay signal it gets from any QR scanner or RFID reader
• Single-use token — each payment opens the gate exactly once
• Token expires after 5 minutes if unused
• Full webhook-to-gate latency target: under 2 seconds
• The ELV technician who installed the turnstile connects two wires — routine work