Praying on a High Place
One may not pray while standing on top of a bed, chair or bench. This rule applies even if these items are not higher than three tefachim (between 24 and 30 centimeters). Nor may one stand on a high place (an area that is more than three tefachim higher than the rest of the ground) unless he is elderly, sick or does so in order for worshipers to hear his voice better.
If this area that is higher than three tefachim is at least four amot by four amot (between about 2 and 2.4 meters), it is considered like a second floor and it is permissible to pray there. Even if it is not four-by-four amot, but the area is surrounded by walls, it is also permissible to pray there since the walls hide the fact that it is higher due to the separation that they create. (Shulchan Aruch Orach Chaim 90:1).
The Piskei Teshuvot explains that there are three reasons given for this prohibition: 1) Fear: One might be afraid of falling and will therefore not be able to concentrate on his prayer (Beit Yosef in the name of Rav Yitzchak Abuhav). 2) Haughtiness: It is written (Tehillim 130:1), “I call out to You (G-d) from the depth (i.e. low places)”, and there is no place for haughtiness before
The Mishneh Berurah discusses the case of an elderly or sick person. Although their standing on a high place is not a sign of haughtiness, nonetheless there still remains the issue (#1 above) of their being afraid and distracted. He gives two answers: 1) In this case, the rabbis were lenient, allowing these people to pray as they prefer, whether seated or standing. 2) The rabbis were lenient only in a case where there is no concern of being afraid of falling.