I would probably put a couple of L brackets on the top, they'd be hidden by the items on the shelf. I once hung a wood shelf over a 5 foot doorway and used only the shelf brackets, but screwed the shelf down onto the brackets as well. I only put very light, non-breakables on it and it held up pretty well.

But in this case, I'd think a couple of L brackets screwed into the studs ought to do the trick.

Thanks, Pesky - makes perfect sense! The wall behind where the shelf would go is board, so it would be very easy to secure the L brackets. Now I just have to decide for sure if I really want to do this, or if I'll start to feel like it's junky (I know it will be a dust magnet) over time. I think it's really cute in the inspiration pic though.

The L-brackets will work but as pesky said you may want to also screw thru the shelf and into the wood supports. Otherwise, as the the L-brackets are screwed into the wall, they may create a gap between the wood brackets and the shelf.
Depending on the design of the supports, you may be able to drill/screw thru them and up into the shelf. If so, that is all you'll need.
But, if you don't want drill into your wall or shelf, just apply some construction adhesive between the top of the brackets and the shelf.....and you're done.

We have two of these types of shelves -- one above the doorways in the kitchen -- and one above the large bay window in the dining room -- and both are secured with large L-brackets on top.

Then the brackets are painted to blend into the wall (plus you can't see them with items in front of them)

Thanks, Annz! I see exactly what you mean.

Oooooh, Teacats - can you share photos? Would love to see your shelves!

You could also put some key holes in the back of the shelf on each end and in the middle and hang it on nails or screws to the wall. This would put less holes in the wall over the door and the shelf will hang securely and no hiding of brackets etc. will be necessary. I really like that shelf idea!

DH built these shelves -- and I painted/crackle-glazed them:

Here's an older photo of the shelf in the kitchen:

I'll try to take some updated photos --including one of the dining room shelf too! :)

Thanks so much, Sasafras!

Teacats, I love your shelf! I like yours even more than I like the inspiration pic! About how deep is yours? It doesn't look very deep, but obviously it holds some fairly good-sized items. Sooooo charming!

Thanks!!! :) You really made my day!!! :)

DH made each shelf about 8 1/2 inches deep -- and then added the trim -- actually parts normally used for adding crown molding to ceilings.

Not sure how your woodwork/trim is shaped, but this is what we did. Our woodwork is flat on the top edge. So we positioned the shelf board on top of the trim so that it rests on the top edge all the way along the back edge. Then we added the brackets at the corners. We also have to deal with vibrations as there are train tracks only 1 1/2 blocks away. So I used rubberized shelf liner under the items on the shelf so they wouldn't vibrate to the front or fall off.

Our daughter had loads of stuffed critters when she was younger so when she outgrew them but still wanted them around I installed an eight in deep shelf most of the way around her room just above the doorway height using decorative brackets both below and above the shelf where needed to hold them in place.

Wonderful shelf, Marylu! I love how seamless it is - how it looks as though it's just a part of the window casing. Having done a more careful look at the area I've considered for the shelf, I'm not now so sure it will work, as brackets would interfere slightly with opening of one of the kitchen cabinets. I'm going to have to think/scheme on it a bit more!

Justgotabme, I'll bet the "critter shelf" in your daughter's room was just adorable! :-)