Codex
Veering away from actually building your wall like defence in your wild when you have Virtus Codex, it is important to defend with a range of different troops.
A standard defence would consist of
1000 conscripts
500 legionaries
200 cenurians
500 skirmishers
200 scorpios
500 cavalry
This defence would give you the advantage as most players attack with a single type of troop against wilds (usually cavalry) so your variation of range will increase your chances of success. It is important to build wild defence when you can but until they are level 10 wilds it is rather pointless because, after all, your wild endgame is to have nothing but level 10 wilds.
Although in your cities you will concentrate on sling bullets for your walls' field defence, your wilds' field defence should consist more of spikes due to the fact that cavalry are used more often than not to take wilds.