All good tips here (except the nerd poles, Loooosers

), @Kalarro don't give up all hope yet, even though we've all got the same basic tips for surviving at least the initial hordes I am starting to see possibilities for interesting designs.
@Scyris is absolutely right that barbed wire is king now.
I haven't gotten a chance to try out my checkerboard support idea yet, have mostly been playing MP now. We just went through 2nd horde with minimal damage, even though there were a lot of cops and even some wights. The server we play on has 2 hour days (le sigh) so it was more like a week 3 horde.
RNJesus blessed us with an amazing trader, with all 4 workstations functional, so we setup shop in the nearby 2 story wood cabin (we joined server on day 3 or 4) for first horde we just fortified the wood walls as best we could, removed the stairs and raised the floor up to 5 blocks high of wooden frames, cut doors out and 2 rows of frames around the outside upper area, hatchevator a few blocks away for access, and I got a single row of spikes around it. They did wreck most of 1 corner and part of the central pillar we added, but didn't bring the base down. There we 4 of us actively defending.
For the 2nd horde I managed to get the bottom 3 blocks all concrete, corners and here and there all the way up in reinforced, rest was at least cobble. Also got barbed wire fences as a ring around the spikes.
Like I said a good # of cops and some feral wights, some radiated spiders & tourists, dogs and the usual trash. Don't think there were any other specials. They actually wrecked the hatchevator for some reason, cop vomit took out a good amount of outer frames, and 1 got close in and blew and took a good chunk of durability from 1 of the reinforced corners.
Main horde was dead by 23:00 and we just had a trickle the rest of the night (I'm wondering if the server had even patched to 199 since I thought trickle was removed...I hope they patch to 208 today).
I only had to replace 3 concrete blocks, and they were actually on different sides of the house. All the spikes were gone (which actually makes me happy since that means they all did their job...also I didn't have to chop them up to replace them with barbed wire for next week). An electric fence perimeter too if I can manage the posts. Electric fence is still hands down my favorite, not only does it hold them in place but it illuminates them too, even this slow old man can rack up kills with a compound bow then.
Granted it is a hell of a lot easier with multiple players when you can all concentrate fire on the specials. Only 1 cop managed to blow, the rest we gangraped.Same for the wights. Solo I'd have been crying like a little girl over my destroyed base and 1 hr of death debuff no doubt (but then the horde probably would have been weaker).
Now that I've got the shell all r-crete I am probably going to start laying down the checkerboard of columns or a maze (still undecided) for the interior of the 1st floor, and if I can talk the guys into it, an actual ladder up in the center, iron bars for the walkways are a priority now as well, since I expect many cops next time. How are the new wooden bars btw?
Since I'm already looking to the future I've been thinking exactly as @Oddux, go tall. Our current main floor would be entirely fighting platform, and build up another few stories so fallback position on the 3rd floor, our "base" on the 4th, swimming pool & garden on the roof...or not. After that I'm thinking of making a maze around the base.
I prefer not having to maintain seperate 6 & 7 day bases. Normally I build my day to day base & mine underneath my horde base cause I am lazy and it was the best of both worlds, but that seems like a non-starter for A17. We may try sinking a mine to bedrock through the center.