Snagov monastery church is built following a plan Athonite. It is the only medieval church in Romania constructed in such (late replicas begin to appear between the two world wars).
The type of plan called Athonite (from Mount Athos) reproduces the spatial-constructive disposition of the church ("catholikon") of Great Lavra. We can name the Athonite type as "Greek cross plan with aisles"
What constitutes this space? Imagine two half-cylinders, of equal length that intersect to form a cross with equal arms (hence the name of a Greek cross). This cross is inscribed in a square (the church body). The middle cross is surmounted by a vertical cylinder (dome Pantocrator) over a spherical cap. This tower download on 4 pillars or columns and the 4 small spaces (squares) remaining at the corners of the big square (the church body) in which is inscribed cross, usually much smaller than the rest of the church, can be covered with small domes or towers. This is a Greek cross church.
Now, imagine that, corresponding to each arm of the cross, outdoor, we join each arm to a vertical half cylinder covered with a spherical cap ( an apse). This is the (absolute) Athonite model. In reality, we have only three apses, on the 4th side joins the narthex. Most times the church has five domes - one large, central and four small at corners: but it's not a strict rule and at Snagov is not respected.
The lateral apses occur because it is a monastery church The Orthodox liturgy (very large, it takes about 3 hours, with many songs, singers are monks willing in the two semicircular apses, they need to see each other to know when get (in the music).That's why all (old) churches with lateral apses can be suspected to have been a monastery church.
What you see is the Pantocrator dome of the nave, where it intersects four half-cylinder and see where heavier four octagonal pillars that support the dome.
Unfortunately the picture was made about the flight, the painting was then in process of restoration. (That's why I have not any photo of the extraordinary painted gallery of the nearly all the princes of Wallachia (the Basarab dynasty)
The tower is white, unpainted, because was restored as falling in one of the many large earthquakes that have shaken Romania. The four wooden ties take some of the lateral thrust of the vaults.