Colossus, a Java version of Avalon Hill’s Titan. I haven’t played it in a few years, but it was pretty good. Certainly played much faster than the board version did.
Oasis, where you search to build a city so you can fend off barbarians. I didn’t actually like it very much, others did and it fits the criteria.
There’s lots of tower defence games at http://www.kongregate.com/strategy-defense-games but there’s also Rebuild and Rebuild II, which are similar to strategy board games. The first one was harder.
On the off chance that you haven’t played Settlers of Catan, it’s a good board game and I’ve played a couple different PC versions.