For those who still fear so much of Russian, Во is used before:
- в, ф + consonant (во Владимире).
- forms of лев, лёд, лён, лоб, ложь, мох, ров, рожь, рот (во льду, во лжи, во рту),
- мне and forms of многие, многое, множество, множественный (во мне, во многом)
- что (во что)
- also во CAN be used with: благо, весь, глава, двор, дворец, дворянство, зло, мгла, мнение, мрак, сколько, слава, сон, спасение, столько, тьма, христос, цвет, человек, чрево (во благо, во весь рост, во сколько, во сне), here sometimes в is more common, sometimes во is, it depends on context.
In this case looking at google в дворце is more used as: в Дворце Спорта, в Дворце Профсоюзов etc. I'm not sure if it's a mistake to use otherwise, it doesn't seem to me.
There are many Russian words where an о or an е can appear or disappear when the word is changed e.g. by conjugating or inflecting it.
For example, звать "to call" but зовут "they call" with an extra -о-; дворец but во дворце where the -е- disappears when the ending is added.
There are historical reasons for the appearance or disappearance of these letters but in the modern language it's probably easiest to just learn the words individually.
A bit like the vowel changes in Latin in some words which English inherited, e.g. efficient versus effective or pronounce versus pronunciation.
In short, castles are fortified and their primary goal is to be able to withstand an enemy attack
Yes and no. Castles are fortified, but their primary purpose was/has been to house some nobility. А fortress=крепость is the one whose primary purpose is to be able to withstand an enemy attack and for that reason they are typically far better fortified in comparison to your typical castle.
"There is no room" is a standard expression indicating just that (i.e. no physical space available). There is no place", on the other hand, is a literal translation from Russian - you will be understood, but you will sound foreign. The problem is that Russian "место" means both a location and available physical space. In English "place" corresponds to the first meaning while "room" (uncountable!) corresponds to the second.
"There is no place" is often used as a part of different common fixed expressions: There is no place like home; there is no place to go.
I think the reason is that во is a preposition, which almost merges the next word when they are pronounced - and that's precisely why it's unstressed. "Но" is not a preposition, moreover it is used to contrast two things/concepts. It can be really stressed, but even when it's not fully stressed, it is still far more stressed than "во".
Also, if you think about it, the only reason prepositions "во", "ко", "co" even get a vowel is not to pile up too many consonants: their "parent", main forms are "в", "к", "c". In order to vocally separate consonants, you don't really need stressed vowels - and so they don't get stressed.