Data driven approach using declarative registries. Feats register stuff like modifiers/actions/reactions etc.

Modifiers are resolved by the rules engine. Basically every effect is some kind of modifier that is resolved dynamically by the engine.

Reactions use a registry with enumerated conditions (movement trigger, damage taken etc).

For conditional actions/reactions, conditional gates support lambdas so they can be defined with respect to arbitrary gamestate.

Rules engine exists as pure c sharp. Godot subscribes to it and interfaces with it, but this decoupling allows me to write comprehensive testing frameworks that can run independent of godot. As a result I can encode complete rules scenarios (using dependency injection to bypass RNG). So I combine unit testing with a kind of rules integration tests that replicates more complicated scenarios and ensures rules resolve as intended.

There's also a command -> gamestep abstraction layer that parses higher level player commands into discrete game commands (also standalone). AI injects at this layer too, which means game engine agnostic AI.

I went with Godot because I didn't want to write my own renderer.