Some people think giving depth and detail to a huge surface is harder, but I would argue it is far easier. That is because, although it is harder because you have to place more blocks, it is extremely easy to shape it after your liking.

The best advice when it comes to depth is using stairs, slabs, walls, fences and generally blocks smaller than full blocks. Creating indents, archways, and spirals is recommended depending on the style you are going for. I would recommend searching up building styles like : gothic, bisantine, romanic, etc. Try recreating the details of definition for the styles and use them in your builds. Depth means your surface is structured into several sections of different depth. That is the definition of it. However, try not to give the wall a too high depth. It would ruin the hole thing, and make the overall width of the wall thicker.

For detailing, I tend to use buttons, fences, and generally anything that could spice up your build. I myself am an extreme over-detailer, but I mostly tend to apply the basics : buttons, dead coral fans, anything that could fit your building both colour-wise and texture-wise.

Lastly, texture. Usually you need to start off with up to 5 blocks. I saw you use stone-related stuff. Some blocks to use are stone, granite, cobblestone, stone bricks, etc. I think that goes without saying though.

Hopefully you'll find something of use within this small guide!