Quote from Longstreet69 >>

Hmmm....apart from the Magic Missile immunity this spell seems pointless for the originating attack. I quote "which you take when you are hit by an attack ". If you react to the hit then the bonus to AC does not count ? Am I missing something here ?

 the spell specifically states that you apply the bonus to the trigger as well as subsequent attacks until the start of your next turn. "including against the triggering attack"
example: you have AC of 15 > you are hit by a roll of 17 > trigger shield > re-check your new AC > attack now misses instead of hits.

It's not easy to visualise how this would happen in game. i like to imagine its like you can see the attack coming, and you KNOW you cant physically move fast enough to dodge or whatever, so instead you use your mental reflexes to whip up a ward and hopefully its enough to block the impact.

Hope that helps

Quote from Sigvard_Vigrid >>

If you cast a bonus action spell, the answer is no. If you cast a standard action spell, then moved (which triggered the attack of opportunity), this could still be cast within the same turn. The cantrip only rule is only given in reference to bonus action spells. 

 I don't believe this is true. I believe the rule is that you can only cast one spell of first level or higher as a part of your standard action economy, so that means either a first level or higher spell as an action or first level or higher as a bonus action. You can cast a cantrip as the other action in either case. Reactions are not a part of that economy, though, so regardless of spells cast on your turn, you can cast this as a reaction at any time that you have a reaction available.

As long as you still have your reaction that round, yes. 
Spells that require reaction do not fall under the rules of one spell per Round

I would also think that some degree of awareness should come into play. I would cry foul if my invisible rogue sneak attacks the enemy wizard from behind and even though he had no idea I was there he could throw up a Shield spell and negate my hard work. (I like to remember that anything that works for the PC's also works for the NPC's as well, so we have to be careful how we decide some of the rules work)

@AgentDraakis

There is already a mechanism for this. A surprised foe (one who did not expect a fight) cannot take action or move on their first turn of combat. Additionally they cannot take reactions until that first turn ends. Better have high initiative!

Question from long ago but in reply I use Shield at higher slots as an abjuration school Wizard as it recharges a damaged arcane ward for more Hit Points.

I use this and counterspell as the main methods of doing this a lot and also extend the ward to the good 'ole party fighter to help them tank (later game that is).

I have cast it as high as 8th level before to get 16hp back onto my arcane ward.

Selecting it as one of my "at will" level 1 spells at higher wizard levels also means I basically have +5 AC under most circumstances and will always heal the ward a minimum of 2hp when used.

The bonus is for when you know you're going to be hit, it's primarily meant to block the triggering attack.

I dont believe so but I'd suggest checking the book for a rules clarification 

i would say no because attack of opportunity is the reaction to you moving in the reach of an enemy

you gain a +5 to the whole round meaning its harder to hit you. But if i had to choose between this and mage armor than id always go mage armor

I have a lot of players who use this spell but I think id rather have mage armor and keep my reaction for anything else

I know the range on this thing is self. But say you were wrapped around or holding onto someone else, could you cast it and trick the spell into wrapping around the two of you? Cause I figured if someone as small as a grung could do it and someone as big as a giant could do it, then maybe you could trick the spell into enveloping more or “you”? Because size isn’t the restricting factor.

so i can't cast this on someone else right?

even with sorcerer metamagic, i mean

thats not what its for, you take it when you are hit, and then if the +5 would make it so you werent hit, you dont get hit anymore.

im assuming no but, if i was to own the staff of defense as a circle of the moon druid would i be able to use 2 out of the 10 charges for shield as a reaction while in a wildshape, or do i still need to cast the spell verbally? (im only level 8 so i cannot cast spells in wildshape yet). 

Twinned spell specifically says the spell cannot have a range of self, so no.

Newish player here so forgive me if this is a question asked incorrectly, but do I need to use a turn to cast this spell (which my DM made me do) or is it only done as a response to have an attack against me? I'm very confused!

Can you cast this using higher level spell slots?

Quote from RSAL >>

Newish player here so forgive me if this is a question asked incorrectly, but do I need to use a turn to cast this spell (which my DM made me do) or is it only done as a response to have an attack against me? I'm very confused!

 It's your reaction with the trigger being your PC being attacked.  You get one reaction per round and it can be spent on a lot of things, for example Attacks of opportunity, Counterspell, Featherfall. Feats like Sentinel and  Many classes have specific abilities that work based on reactions like a Rogue's uncanny dodge, Bard's cunning word, etc.  Reactions refreshes at the start of your turn. Reactions can be used at any time during anyone's turn as long as the qualifiying trigger happened.  So your DM is wrong. 

Quote from AlexParys >>

Can you cast this using higher level spell slots?

 

 You can upcast any spell, but it gains no additional benefits unless the spell states otherwise.