diff --git a/defs/data.def b/defs/data.def
new file mode 100644
index 0000000..3474cb7
--- /dev/null
+++ b/defs/data.def
@@ -0,0 +1,20 @@
+[SITE:CAVE]
+	[SIZE:1:3]
+	[UNDERGROUND]
+
+[CREATURE:BANDIT]
+	[NAME:bandit:bandits]
+	[HEALTH:8]
+	[AC:12]
+	[ATTACK:2:1:8]
+	[HUMANOID]
+	[PATROL]
+
+[CREATURE:SPIDER]
+	[NAME:spider:spiders]
+	[HEALTH:12]
+	[AC:13]
+	[ATTACK:2:3:10]
+	[MULTIATTACK:2]
+	[ANIMAL]
+	[PREFER_DARK]
diff --git a/src/definitions.rs b/src/definitions.rs
new file mode 100644
index 0000000..7972ab4
--- /dev/null
+++ b/src/definitions.rs
@@ -0,0 +1,265 @@
+use std::{borrow::BorrowMut, io::Read, str::FromStr};
+
+#[derive(Debug)]
+pub enum TopLevelTag {
+    Site,
+    Creature,
+}
+
+#[derive(Default, Debug, Clone)]
+pub struct Tag {
+    pub name: String,
+    pub values: Vec<String>,
+}
+
+impl Tag {
+    pub fn parse_value<T: ParseQuery>(&self) -> Result<T, ParseError> {
+        T::parse(&self.values).map_err(|_| ParseError::Parser(format!("Invalid value {:?} in tag {}", self.values, self.name)))
+    }
+}
+
+pub trait ParseQuery: Sized {
+    fn parse(values: &Vec<String>) -> Result<Self, ()>;
+}
+
+trait Parseable: Sized + FromStr {
+    fn parse(string: &str) -> Result<Self, ()> {
+        string.parse().map_err(|_| ())
+    }
+}
+
+impl Parseable for String {}
+impl Parseable for i32 {}
+impl Parseable for f32 {}
+impl Parseable for bool {}
+
+impl<A: Parseable> ParseQuery for A {
+    fn parse(values: &Vec<String>) -> Result<Self, ()> {
+        Ok(A::parse(values.get(0).ok_or(())?)?)
+    }
+}
+
+impl<A: Parseable, B: Parseable> ParseQuery for (A, B) {
+    fn parse(values: &Vec<String>) -> Result<Self, ()> {
+        Ok((
+            A::parse(values.get(0).ok_or(())?)?,
+            B::parse(values.get(1).ok_or(())?)?,
+        ))
+    }
+}
+
+impl<A: Parseable, B: Parseable, C: Parseable> ParseQuery for (A, B, C) {
+    fn parse(values: &Vec<String>) -> Result<Self, ()> {
+        Ok((
+            A::parse(values.get(0).ok_or(())?)?,
+            B::parse(values.get(1).ok_or(())?)?,
+            C::parse(values.get(2).ok_or(())?)?,
+        ))
+    }
+}
+
+#[derive(Default, Debug)]
+pub struct Definitions {
+    pub sites: Vec<SiteDef>,
+    pub creatures: Vec<CreatureDef>,
+}
+
+#[derive(Default, Debug)]
+pub struct SiteDef {
+    pub id: String,
+    pub min_size: i32,
+    pub max_size: i32,
+    pub is_underground: bool,
+}
+
+impl SiteDef {
+    fn parse(header: Tag, tags: &[Tag]) -> Result<Self, ParseError> {
+        let mut result = Self {
+            id: header.parse_value::<String>().unwrap(),
+            ..Default::default()
+        };
+        for tag in tags {
+            match tag.name.as_str() {
+                "SIZE" => (result.min_size, result.max_size) = tag.parse_value().unwrap(),
+                "UNDERGROUND" => result.is_underground = true,
+                _ => {
+                    eprintln!("Unknown tag '{}' in SITE defs", tag.name);
+                }
+            }
+        }
+        Ok(result)
+    }
+}
+
+#[derive(Default, Debug)]
+pub struct CreatureDef {
+    pub id: String,
+    pub name_singular: String,
+    pub name_plural: String,
+    pub health: i32,
+    pub armor_class: i32,
+    pub to_hit_modifier: i32,
+    pub min_damage: i32,
+    pub max_damage: i32,
+    pub multiattack: Option<i32>,
+    pub is_humanoid: bool,
+    pub is_animal: bool,
+    pub patrols: bool,
+    pub prefers_dark: bool,
+}
+
+impl CreatureDef {
+    fn parse(header: Tag, tags: &[Tag]) -> Result<Self, ParseError> {
+        let mut result = Self {
+            id: header.parse_value().unwrap(),
+            ..Default::default()
+        };
+        for tag in tags {
+            match tag.name.as_str() {
+                "NAME" => (result.name_singular, result.name_plural) = tag.parse_value()?,
+                "HEALTH" => result.health = tag.parse_value()?,
+                "AC" => result.armor_class = tag.parse_value()?,
+                "ATTACK" => (result.to_hit_modifier, result.min_damage, result.max_damage) = tag.parse_value()?,
+                "MULTIATTACK" => result.multiattack = Some(tag.parse_value()?),
+                "HUMANOID" => result.is_humanoid = true,
+                "ANIMAL" => result.is_animal = true,
+                "PATROL" => result.patrols = true,
+                "PREFER_DARK" => result.prefers_dark = true,
+                _ => {
+                    eprintln!("Unknown tag '{}' in CREATURE defs", tag.name);
+                }
+            }
+        }
+        Ok(result)
+    }
+}
+
+pub enum ValueToken {
+    String(String),
+    Integer(i32),
+}
+
+#[derive(Debug)]
+pub enum ParseError {
+    Lexer(LexerError),
+    Syntax(String),
+    Definition(String),
+    Parser(String),
+    Unexpected,
+}
+
+#[derive(Debug)]
+pub enum LexerError {
+    Io(std::io::Error),
+    UnclosedTag,
+}
+
+pub struct DefinitionParser;
+
+impl DefinitionParser {
+    pub fn parse(source: impl Read) -> Result<Definitions, ParseError> {
+        let mut defs = Definitions {
+            sites: vec![],
+            creatures: vec![],
+        };
+
+        let tags = match Self::lexer(source) {
+            Ok(tags) => tags,
+            Err(err) => {
+                eprintln!("{err:?}");
+                return Err(ParseError::Lexer(err));
+            }
+        };
+
+        let mut unparsed_defs: Vec<(Tag, Vec<Tag>)> = Vec::new();
+
+        let mut definition_builder: Option<(Tag, Vec<Tag>)> = None;
+        for tag in tags {
+            match Self::match_top_level_tag(&tag.name) {
+                Some(_) => {
+                    if let Some(builder) = definition_builder {
+                        unparsed_defs.push(builder);
+                    }
+                    definition_builder = Some((tag, vec![]));
+                }
+                None => match definition_builder.borrow_mut() {
+                    Some(builder) => builder.1.push(tag.clone()),
+                    None => {
+                        return Err(ParseError::Syntax(
+                            "First tag must be a top-level-tag".into(),
+                        ))
+                    }
+                },
+            }
+        }
+
+        match definition_builder {
+            Some(builder) => unparsed_defs.push(builder),
+            None => {
+                return Err(ParseError::Definition("No definitions found".into()));
+            }
+        }
+
+        for (header, tags) in unparsed_defs {
+            match Self::match_top_level_tag(&header.name) {
+                Some(def_type) => match def_type {
+                    TopLevelTag::Site => defs.sites.push(SiteDef::parse(header, &tags)?),
+                    TopLevelTag::Creature => {
+                        defs.creatures.push(CreatureDef::parse(header, &tags)?)
+                    }
+                },
+                None => {
+                    return Err(ParseError::Unexpected);
+                }
+            }
+        }
+
+        println!("{defs:?}");
+
+        Ok(defs)
+    }
+
+    fn lexer(mut source: impl Read) -> Result<Vec<Tag>, LexerError> {
+        let mut tags = vec![];
+        let mut tag_builder: Option<Tag> = None;
+        let mut buffer = String::new();
+        source
+            .read_to_string(&mut buffer)
+            .map_err(|err| LexerError::Io(err))?;
+        for c in buffer.chars() {
+            match tag_builder.borrow_mut() {
+                None => match c {
+                    '[' => tag_builder = Some(Tag::default()),
+                    _ => {}
+                },
+                Some(tag) => match c {
+                    ']' => {
+                        tags.push(tag.clone());
+                        tag_builder = None
+                    }
+                    ':' => tag.values.push("".into()),
+                    '[' => return Err(LexerError::UnclosedTag),
+                    _ => {
+                        if tag.values.is_empty() {
+                            tag.name.push(c)
+                        } else {
+                            tag.values.last_mut().unwrap().push(c)
+                        }
+                    }
+                },
+            }
+        }
+        if tag_builder.is_some() {
+            return Err(LexerError::UnclosedTag);
+        }
+        Ok(tags)
+    }
+
+    fn match_top_level_tag(tag: &str) -> Option<TopLevelTag> {
+        Some(match tag {
+            "SITE" => TopLevelTag::Site,
+            "CREATURE" => TopLevelTag::Creature,
+            _ => return None,
+        })
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index 3d8ec24..522f465 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -2,7 +2,6 @@ use std::{
     collections::{HashMap, HashSet},
     fmt::Display,
     num::NonZeroUsize,
-    ops::Sub,
     time::Duration,
 };
 
@@ -10,6 +9,8 @@ use enum_dispatch::enum_dispatch;
 use serde::{Deserialize, Serialize};
 use uuid::Uuid;
 
+pub mod definitions;
+
 const SAVE_FILE: &'static str = "world.bin";
 
 fn main() {
@@ -30,24 +31,27 @@ fn main() {
         }
     };
 
-    for site in world.sites.iter() {
-        println!("{site}")
-    }
+    let source = std::fs::File::open("defs/data.def").unwrap();
+    definitions::DefinitionParser::parse(source).unwrap();
 
-    let mut input = String::new();
-    loop {
-        std::io::stdin().read_line(&mut input).unwrap();
-        if vec!["q", "quit", "exit"]
-            .iter()
-            .any(|quit_cmd| input.trim() == *quit_cmd)
-        {
-            break;
-        }
-        let start = std::time::Instant::now();
-        world.advance_time(Duration::from_secs(3600));
-        let end = std::time::Instant::now();
-        println!("World tick: {}us", end.sub(start).as_micros());
-    }
+    // for site in world.sites.iter() {
+    //     println!("{site}")
+    // }
+
+    // let mut input = String::new();
+    // loop {
+    //     std::io::stdin().read_line(&mut input).unwrap();
+    //     if vec!["q", "quit", "exit"]
+    //         .iter()
+    //         .any(|quit_cmd| input.trim() == *quit_cmd)
+    //     {
+    //         break;
+    //     }
+    //     let start = std::time::Instant::now();
+    //     world.advance_time(Duration::from_secs(3600));
+    //     let end = std::time::Instant::now();
+    //     println!("World tick: {}us", end.sub(start).as_micros());
+    // }
 }
 
 #[derive(Debug, Default, Serialize, Deserialize)]
@@ -113,7 +117,7 @@ impl Site {
                     .get_mut(index)
                     .expect("Creature generation index for depth is out of bounds")
                     .population
-                    .push(Creature::new(CreatureData::Bandit(Bandit::default())))
+                    .push(Creature::new(CreatureBehaviour::Bandit(Bandit::default())))
             }
         }
         {
@@ -126,7 +130,7 @@ impl Site {
                     .get_mut(index)
                     .expect("Creature generation index for depth is out of bounds")
                     .population
-                    .push(Creature::new(CreatureData::Spider(Spider::default())))
+                    .push(Creature::new(CreatureBehaviour::Spider(Spider::default())))
             }
         }
     }
@@ -188,7 +192,7 @@ pub struct SiteArea {
 #[derive(Clone, Eq, PartialEq, Hash, Debug, Serialize, Deserialize)]
 pub struct Creature {
     pub uuid: Uuid,
-    pub data: CreatureData,
+    pub data: CreatureBehaviour,
 }
 
 impl Display for Creature {
@@ -198,7 +202,7 @@ impl Display for Creature {
 }
 
 impl Creature {
-    pub fn new(data: CreatureData) -> Self {
+    pub fn new(data: CreatureBehaviour) -> Self {
         Self {
             uuid: Uuid::new_v4(),
             data,
@@ -206,7 +210,7 @@ impl Creature {
     }
 }
 
-#[enum_dispatch(CreatureData)]
+#[enum_dispatch(CreatureBehaviour)]
 pub trait CreatureImpl {
     fn species_id(&self) -> String;
 
@@ -223,11 +227,21 @@ pub trait CreatureImpl {
 
 #[derive(Clone, Debug, Eq, PartialEq, Hash, Serialize, Deserialize)]
 #[enum_dispatch]
-pub enum CreatureData {
+pub enum CreatureBehaviour {
+    Default(DefaultBehaviour),
     Bandit(Bandit),
     Spider(Spider),
 }
 
+#[derive(Clone, Debug, Default, Eq, PartialEq, Hash, Serialize, Deserialize)]
+pub struct DefaultBehaviour;
+
+impl CreatureImpl for DefaultBehaviour {
+    fn species_id(&self) -> String {
+        "default".into()
+    }
+}
+
 #[derive(Clone, Debug, Default, Eq, PartialEq, Hash, Serialize, Deserialize)]
 pub struct Bandit {}