Check out tho documentation of cat you can specify the dimension along which you want to concatenate N-dimensional arrays:
1D:
julia> append!([1, 2, 3], [4, 5, 6]) # mutating!
6-element Array{Int64,1}:
1
2
3
4
5
6
julia> cat(1, [1, 2, 3], [4, 5, 6])
6-element Array{Int64,1}:
1
2
3
4
5
6
Generalization:
julia> cat(2, [1, 2, 3], [4, 5, 6])
3×2 Array{Int64,2}:
1 4
2 5
3 6
julia> cat(3, [1 2 ;3 4], [5 6 ; 7 8])
2×2×2 Array{Int64,3}:
[:, :, 1] =
1 2
3 4
[:, :, 2] =
5 6
7 8