My experiences so far - Navezgane, nomad difficulty, solo. Defending with a bow and arrows, mostly.
Zombies smash through early walls pretty quickly (a little too quickly, I feel), and spikes don't last long (iron or wood).
I figured doors would be a magnet and would focus the power of the horde too much. So I built a solid 5 X 5 X 5 tower of wood/iron for Day 7, with a parapet. This spread them around a bit more and diluted the damage, especially if I kite them around the perimeter from above.
It's accessed by two short elevated runs with trapdoors as 'mini-drawbridges' and wooden doors to discourage any Z's trying to jump the gaps. I've found that they don't try to gain access by the elevated walkways - maybe they don't recognise it as an entrance or maybe the ladder, doors and two jumps required aren't as attractive as going straight for the base.
Instead, they go straight for the tower and go into area-destruction mode. I had 3 layers of spikes around the tower on day 7, and that became 5 layers of iron spikes, wood spikes, and barbed wire on day 14.
I lost half the tower on day 7, but the upper platform survived. On day 14 the lower layers had been upgraded to near max scrap iron and it survived a lot better. Then again, I had 3 cops turn up on day 14 (which seemed a bit early) and I died because they destroyed the wood frame parapet and I didn't notice before I was falling

Not sure what my game stage was before but after that death it was 60. Also I had to do a quick relog because of something that spammed constant error logs in the console - happens at 05:55 ish in the Day 14 video below.
So I survived Day 7 without a scratch and died once on Day 14 but was otherwise quite comfortable. The base survived both nights.
Thinking about my experiences and the OP's, I think I'll stick to not having any sort of single entrance that the Z's can recognise.
Evidence - sorry, they are a bit choppy:
Day 7 -
Day 14 -
On another note - underground digging seems to be completely dead. In previous Alphas I've always had some sort of underground base, though I never stayed down there on a horde night. I just tried digging down 30 blocks below ground level to build an item store. But on the last night I played, a wandering horde ran up to the edge of my tower and burrowed straight down to me. So if they can 'see' me through the ground even when it's not a horde night, there's no point digging down anymore. But I did learn something positive - they don't seem so good at going up again - the buggers are still down there