A few notes on defence.
1. On reserves, check Land Ops at 7-58 to 7-60. The reserve preserves a commander's freedom of action through initial uncommitment. That doesn't mean you do not assign it potential tasks - to put it another way, if you have an element that is outside of your main and supporting efforts and is given a series of "Be prepared to" tasks, then you've constituted a reserve. Conversely, if you put a unit in depth and give it a "block" task, then it isn't really a reserve, as by executing his task to block, that subordinate isn't really giving you freedom of action.
In fact, counter-moves (reinforcement/blocking/counterattack) is the likely probable task of reserves in the defence. Historically, good defensive schemes put a large proportion of their strength in depth reserves which were tasked with counter-moves. A good example that I recently ran into was Mustafa Kemal at Gallipoli. His division held a sector in which he placed two of his regiments up, retaining a third in reserve. As well, he took one of three battalions from each of the frontline regiments into the reserve as well: in total, five of his nine battalions were reserves.
2. On reserves. Sometimes, I don't think we really put thought into what constitutes our reserve - we just designate one to check the box, even if it doesn't make sense. I'm personally not convinced a platoon needs a reserve. In many (most?) cases, a company probably doesn't need one either. What does a company commander have that can preserve his freedom of action? Can an 8-man section really do much for him? If the nature of the task and terrain allows a company commander to keep a 1/2 platoon or greater out of the battle, then I think he has something workable. I think its entirely suitable for a battalion commander to carve off parts of his companies to form a central reserve, and for the company commanders to have none.
3. My personal belief is that the Canadian Army doesn't "do" defensive exercises too well as it isn't in our "genetic structure." Over the last 100 years, we spent the First World War attacking Germans on the Western Front, spent the Second World War attacking Germans in Italy and NW Europe, maintained a war of patrols with an unmechanized foe in Korea, and spent the Cold War in Europe preparing to fight a form of mobile defence. From my reading on 4 CMBG, we likely achieved a degree of sophistication in mechanized defence there, but that's about state of the art for us.
Our theory and our operational approach pales into anything you could find in reading about the Gustav, Gothic, or Siegfried Lines. Look at the typical battalion defensive exercise. The battalion goes out, puts three companies on line, and then digs a fairly linear position. Brigade defensive schemes I've observed on exercises aren't much more sophisticated. The general concept is that we wait (and hope) the enemy bangs into our obstacles and gets destroyed by our fires while trying to breach them...how is it we always breach enemy obstacles but they really get hung up on ours? There is no depth, no understanding of the immediate and deliberate counterattack, and no designation of battlespace into zones/sectors to fight a phased battle. Find an officer who can put a brigade in an elastic defence.
Anyone who wants to really understand the defence needs to read Lossberg's Memoirs
(now in English!). He essentially invented the modern system of defence in depth that is still predominant today - he was the guy who patched things up in Arras after the Canadian Corps blew a hole open at Vimy.