I'd tried before to play a game where I only allow myself my single starting city. You can build armies, capture lairs, towers, nodes, and attack enemy cities, but any city you take must be razed and you cannot build settlers.
Last time I tried this I was on the defensive the whole time, and held out until one of the AI wizards cast Time Stop, and then just battered me repeatedly until I had no mana and no units left alive.
I tried again over Easter and won :) It didn't even seem particuarly difficult this time. I picked 11 Life Books so I could get Stream of Life and whack taxes up to full, and built Longbowmen and cast Heroism (only) on them. It took a while to find some weak nodes I could take out to get my power base up, but after a while I mostly left my Longbowmen defending, and had a few sets of heroes wandering around the map destroying any lairs, raiders or enemy wizards they could find :)
Some interesting points I found are that, since you only get one city, if you don't start next to the water then you'll never be able to get boats and so getting to other islands proves interesting. Also since I picked High Elves, I could never build roads. Also with only limited magic power, I had something like 90% diverted into Mana to try to pay for my spell maintenance (and I was still getting -50 mana per turn and only surviving with Alchemy, killing the occasional AI wizard and breaking items on the anvil) so research was VERY slow. I got all 10 very rare spells in the end, but I got all of them from taking lairs. It also means it will take you AGES to research the Spell of Mastery, so you probably have to go for a military victory.
Implode.
Single city
Re. single city
I just did a couple of these, they are fun. The 11 white with SOL (like Implode described) worked very well. I dont use Torin, but I got lucky and got Roland for my first champion, and he basically ruled the world by himself (merchant sold me the wraithform sheild and I got a nasty flaming axe, cast heroism and invul - game over). The 11 green with gaias blessing was much harder. Roland is much > than a basilisk, no matter which way you slice it. Had to turtle like crazy until I could get some gorgons going, then slowly ate at my continent, falling further and further behind....until I took a tower and got summon wyrm....game went pretty quickly after that. Much harder though - I was taken down to 2-3 units in my tower many times, and If I hadnt found that wyrm spell I was definitely going to lose, and was only fighting one wizard, at that. I dont think you can do 1 city without a city spell, you have to have a way to field a couple stacks of troops AND defend your tower, so a non-buffed city just does not provide the money/food. The worst part is watching them take your nodes and knowing you do not have the food to post defenders (just one defender at a node prevents them from sending spirits). Fun stuff.