I talked to my DM about that at my last session and he said you can't cast shield while holding a maul. Specifically, he said I could cast on my turn because I can drop my weapon and then retrieve it afterward with weapon bond, but I can't hold the weapon and cast spells with somatic components at the same time, which is what I'd be doing for the reaction. To do that, I'd need the war caster feat. Otherwise I'd have to leave my weapon on the ground between turns.
When I questioned him on it, he said that one of the designers had specifically clarified the issue. After our session, he sent me the link below.
http://www.enworld.org/forum/content...earls-Crawford
The specific text is about midway through the "Spells and Magic" section, but I'll copy it below so you don't have to dig.
"can you cast a spell that uses somatic components if you weild a 2 handed weapon or a vertile weapon in 2 hands?" nope. -M
That doesn't really affect the defensive calculus, but I thought it was worth pointing out. It kind of seems like the EK really needs to take war caster if he wants to use the shield spell unless he's going to be using the defense or dueling fighting style with no shield. Honestly, that's kind of disappointing. It limits the class even more.