ascend 正式用词，指不用手攀，一直上升直到相当高的地方。
climb 普通用词，含义广泛，侧重运用手足费力地攀登或上升，也可指抽象事物。
mount 书面用词，词义与ascend相近，强调连续不断地向上移动、攀登或上涨。指骑马时用mount。
参考例句
1. The soldiers stood beside their horses, waiting for the order to mount. 士兵们站在马的旁边，等待上马的命令。
2. Bills soon mount up at hotels. 旅馆里帐单很快地增加。
3. Courage is the ladder on which all the other virtues mount. ---- Luce. 勇气是一架梯子，其他美德全靠它爬上去。---卢斯。
英英解释
n.
1. a lightweight horse kept for riding only
2. the act of climbing something
3. a land mass that projects well above its surroundings; higher than a hill
4. a mounting consisting of a piece of metal (as in a ring or other jewelry) that holds a gem in place
5. something forming a back that is added for strengthening v.
1. attach to a support
2. go up or advance
3. fix onto a backing, setting, or support
4. put up or launch
5. get on the back of
6. go upward with gradual or continuous progress
7. prepare and supply with the necessary equipment for execution or performance
8. copulate with
