A stronghold may have a foundation, but a tree has its roots deeply planted in the ground. A stronghold can only take so much punishment, but a tree cam stand firm because of its roots.
A stronghold has walls, inner and outer courts, and tall towers, each desgined and integrated to stand effectively against attacks; however, if you find a weakness, or if you hit it enough times, it will fall. The simple design of a tree entails that there are only a few possible ways of attacking it. You can burn it, freeze it, parch it, infest it, irradiate it, choke it, and/or chop it down, but if you do not take care of the roots, new trees will rise.
The righteous root endures. To God be all the glory!