If training in a controlled environment is your thing then a wall is all you need. Climbing walls can be setup in various kinds of indoor spaces such as multipurpose gyms or auditoriums. They are typically built by fixing resin holds on to wooden boards and can be modified very easily to suit your space.
While indoor walls may not offer the same experience as outdoor walls, accessibility, convenience and practicality are what give them an upper hand.
The drawback? No cheat days due to bad weather!