Well, you just use "terrain - Apply cliff: increase one" to create walls.
Those walls look really bad (just look at your terrain... don't tell me that looks good).
There are other methods of achieving good-looking walls (most higher-quality maps use the Apply Height tool, which only works on 'hacked' world editors, such as JNGP, which enables a 90 degree slope).
Additionally, you should improve all tooltips. Don't say "Start here!", because everyone knows that. Just give either the basic stats, or a short overview of the building.
And make the terrain seem less random. Right now it's "a cliff here, a cliff there... done!", make it allow strategies. Increasing the cliff height in certain areas could be good (easier to defend), while it can have some drawbacks (more entrances), while there's also a lower cliff with only 1 entrance (since you're on low ground, your base is weaker, yet a single entrance is easier to defend).
Maybe additional resources, a way to increase the speed of the game (right now you just wait it out) etc.
Add difficulties! Right now it's really, really hard to play alone.
Do more with your map. You kept yourself to the pure basics, go beyond that.