Sample file should now be correct, added check in level loading.

src/level.zig

@@ -9,6 +9,7 @@ const ray = @cImport({
 });
 const std = @import("std");
 const expect = std.testing.expect;
+const assert = std.debug.assert;
 
 const Vec2 = @import("vec2.zig");
 const Rect = @import("rect.zig");
@@ -58,6 +59,8 @@ pub fn init_load(allocator: *std.mem.Allocator, kble_file_path: []const u8) !Sel
         .selection = undefined,
     };
 
+    const expected_bytes_per_cell = @sizeOf(cell_type);
+
     // Open directory.
     var dir: std.fs.Dir = std.fs.cwd();
     // Open file in read mode.
@@ -66,6 +69,14 @@ pub fn init_load(allocator: *std.mem.Allocator, kble_file_path: []const u8) !Sel
         .write = false,
     });
     defer file.close();
+    // Get reader.
+    var reader: std.fs.File.Reader = file.reader();
+
+    // Read first byte and check than the value matches the size of cell_type.
+    {
+        const cell_spec_byte = try(reader.readByte());
+        assert(cell_spec_byte == expected_bytes_per_cell);
+    }
 
     return self;
 }