Maps have long been invaluable tools for navigation and communication, yet their inherently visual nature poses challenges for people with visual impairments or disabilities. While tactile and acoustic maps address some barriers, the rise of digital and web-based maps has introduced new accessibility challenges. This paper reviews the accessibility of popular web mapping tools, proposes guidelines for the development of accessible thematic web maps, and demonstrates these concepts through the development of a prototype using point data of Austrian castles as a thematic layer. The study reveals that while the Web Content Accessibility Guidelines (WCAG) provide a generic framework applicable to thematic web maps, additional domain-specific recommendations are necessary to address unique challenges. Accessibility features across web mapping tools vary widely, with some offering built-in support and others requiring developers to implement accessibility measures. This work contributes to advancing inclusivity in web cartography, fostering equal access to spatial information for users with visual disabilities.